When writing it's important that your characters not only be understandable but they must also be believable to readers. If their dialogue isn't natural then readers won't feel any sort of connection with them and they'll grow bored and move on.
We humans don't speak properly. We often use slang, curse, and we certainly don't follow the rules of speech to the letter. At times, sentences may seem incomplete to a word processor's spelling and grammar check, but to us, they're completely understandably.
When you're writing dialogue you want it to be believable to readers. If it's stiff and formal readers won't buy into it and they won't want to read it. The characters may end up being flat and boring. Unless your character is visiting from a foreign country and they learned the language in a class that taught them how to properly speak the language, they will speak in a more relaxed fashion.
The following is a conversation in a novel that I've written (ok, partially written, it is sitting unfinished somewhere in my files) Lia's brother, Marcello, is the head of an organized crime family. He has guilted her into remaining at his side and working for him. Now he feels that she has become obsolete, but she knows too much about his operations for him to let her walk away. He has decided to eliminate her. Jerome is her bodyguard and he is determined to keep her alive, even if her brother now wants her dead. They have run from her brother. They're in the car driving to the location where they're going to hide when this conversation takes place.
Lia glanced sidelong at Jerome. "What was it like?"
He frowned at her. "What was what like?"
"Being a member of the police force," she replied.
He grinned. "Ah, checking up on me?"
Though she couldn't see him in the darkness, she was certain his green eyes were dancing with humor. Rome, despite his line of work, tended to lean toward being humorous. She shook her head, and her hair brushed over her shoulders as she did, she wished she'd taken the time to pull it up. It was annoying her and she wanted it out of her face. She grabbed it and dragged it back with a sigh. "No, I'm not. My brother did the checking before he considered hiring you. He wanted to be certain that he wasn't hiring an undercover cop or something. He was a bit nervous when he discovered that you had been on the force."
"Obviously he got past that or I wouldn't be here."
"He spoke with some of the cops he has on his payroll and they confirmed the details his people dug up." She turned red when she noticed he was grinning at her, while he managed to keep one eye on the dark road ahead of them. "Okay, so I may have taken a little peek at his report on you."
He chuckled and turned back to focus on the road. "In answer to your question, I liked being a cop. What I didn't like were the rules that restricted our actions. I made decisions and those decisions got me booted from the force but I stand by those actions."
"And decided that a life of crime suited you better," she answered with a little laugh.
"Not at all. What I decided was that I have skills I can share. Skills that many will pay for. I've had clients on both sides of the law."
"You have?" she asked with a note of shock in her voice.
"Yup, I have. I was a bodyguard for Senator Lee Brandon for a bit during the last election. I'm sure if you Google him you'll find some images of me with him."
"I didn't realize that. I assumed that because you were working for my brother ..."
"That I'm a thug with no morals," he finished for her.
She turned red again. "Yes, I'm sorry. It was a logical assumption. My brother is a criminal."
"I hired on with your brother because he pays well. I'm not one of his hired guns and I'm not aiming to be a hitman on his payroll. I'm here to guard you and keep you safe and alive."
That was exactly what he'd done for the past nearly three years. He'd kept her alive through two attempts on her life in the past six months and even now he was risking it all to keep her safe. Her brother wasn't going to be pleased when he discovered they'd escaped his grasp.
"He'll kill us if he gets his hands on us," she told him quietly.
"Let's make fucking certain it doesn't come to that end."
She nodded and tried to ignore the hard edge in his voice. Rome might be a bodyguard with some morals and a distaste for killing, but she had the feeling he'd shoot her brother without a second thought. Whether Marcello was a direct threat or not. The conversation is relaxed and easy between the two. It isn't formal. You'll find that some spell/grammar checks will want you to avoid contractions, Word is famous for it. But let's be honest, no one talks like that. People tend to be informal in their conversations, especially with people they're comfortable with.
It would have much less impact if Rome told her, "Then let us make certain that it does not come to that end." It's extremely formal sounding and it takes away from the threat behind his words. It sounds too pleasant and polite and you lose the hard force of his words and the threat to her brother if it does end in a confrontation.
A lot of word processors will tell you a sentence is incomplete. For instance when Lia says, "And decided that a life of crime suited you better", Word informed that I needed to change it to, "And then you decided that a life of crime suited you better". Yes, it is a more complete sentence but the original one makes sense as well, it simply isn't as structured as Word wants it to be. Word processors look for proper speech, how you would speak if you followed all the rules, so they don't account for slang or the relaxed way we often speak.
Part of making things believable if accounting for how people normally speak. If the words don't match the intent it may become confusing for a reader. Make certain what you're characters are saying matches their intent and their personality. Rome is a hired body guard and he works for a man who is a criminal, but he isn't a a street thug so his personality differs from others who might work for Marcello. He has a moral code and it will make how he speaks different from how others in the organization might speak. I'm not saying that everyone in Marcello's organization has to be a mobster stereotype, but their background and up bringing will effect how they behave, speak, and act. You characters aren't simply a description of their appearance. You must include a background, family, a job, and education, so they become real to the readers. If you miss a detail they may end up flat and unbelievable. They need reasons for their actions and without those reasons the reader will no longer believe in them.