Most people have certain words and phrases they fall back on. They use them over and over. That gets to be boring when you read books written like that. Vary your word usage because you begin to sound like a writer geared toward elementary school kids
Sometimes I know a word but just don’t use it enough to think of it as I’m writing. The thesaurus reminds me of it. It also gives me ideas that I just didn’t think of. There are words that just don't come to me. The thesaurus comes in handy, but don't look at it as a starting point.
As you are writing, don’t focus too much on using the thesaurus. Get the thoughts out. Use the thesaurus when you go back over it and start your first round of self-edits. See if there are any other words that would sound better. Chances are you’ll find one. Rewriting improves your writing, but initially focus on the story. You can choose better words later.
For example, ‘said’. Too many authors use this a lot. It’s easy to do, but you can evolve your writing into more mature material by using different words. Try: exclaimed, shouted, whispered, mumbled, hinted, answered, responded, or so many others. Use the thesaurus and start looking up new ways to say the same old thing.
Go over your writing. Find words you use repeated. Use the thesaurus to find other words that can make your writing more mature.
Don’t be afraid of using this resource. It is educational and fun, not a scary monster.