So, you’ve chosen your purpose: to inform. And your topic: say you want to write about your dog. Now, you need to decide what to focus on. Do you want to inform people of the benefits of having the same kind of dog you do? Great! Let’s talk about that.
Your dog is a great dog–I’m absolutely certain of that. I mean, my dog is a great dog. She’s calm, even-tempered, not mean in the slightest. She’s territorial, but can tell friend from stranger (and doesn’t go batshit insane when it’s a friend). She’s a great little dog–a Scottish Terrier.
But…she’s stubborn. She’s not easy to train because, while she does want to do things that make her family happy, she also wants to do things she wants to do. She yanks on the leash, and if her front feet are on the ground, she can jerk hard enough to pop my wrist, elbow, and shoulder, and yank me off balance.
So…focus. My purpose is to inform readers of what a great dog a Scottish Terrier is. My focus should be on her good points: that she’s got a great temperment, is very smart, isn’t huge or hyper, and takes minimal grooming. Yes, I probably should mention her bad points, but shouldn’t dwell on them. They should kind of be glossed over. Otherwise…I could end up complaining about the dog’s bad behavior, and turning my audience off of getting a Scotty dog.
How do you stay on focus? Easy: have a good, strong thesis statement that includes your reasons. Those listed reasons can easily be turned into topic sentences for your main body paragraphs. My paper about my Scotty would have a thesis statement that read like this:
A Scottish Terrier is a great dog for a family because they’re even tempered, not too big or hyper if you have a small space, and don’t take much grooming.
My reasons would be used later as topic sentences for body paragraphs, helping me avoid the whole focus pitfall of telling my audience what a great choice a Scotty is, but spending the whole paper complaining about how stubborn she is, and what a pain she can be to take care of–even though that’s not what I think at all.
To recap: focus is important. If you lose sight of what you want to write about, not only will your audience have no clue what you’re trying to say, but also might be convinced of the opposite of what you intended. Your thesis statement is an important part of helping you maintain the focus you wanted.