Terms of Service are an agreement between you and your user. It sets the terms and conditions of the use of your product.
So, you have a website and a user lands on it. They roam around, have a look at your product, and sign up. They pay for using the product.
When signing up (or using it without signing up), you want to set certain conditions about the use of the product. That’s where ToS come in handy.
With the ToS you tell the user: “Hey user, I’m glad you are signing up, but you need to accept these terms and conditions. I can’t let you use my product if you don’t accept my terms. Take it or leave it.”
If you don’t offer a product that requires signing up (you offer a free product, website content, etc) you may want to set the terms and conditions for using your website.
You tell the user: “Hey, if you keep using this website/free product, it means you accept to use it according to the terms I set. This is my website and I want my users here to behave in the way I want. If you stay here it means that you accept my terms.”
This is non-negotiable.