They negotiate for YOU...that's what they do.
The commission is paid by the seller. While this isn’t great, if you are the seller, just think of all the backlash you could get if a sale goes south when the buyer finds something they don’t like while you are in escrow. Agents help prevent this.
They literally do this for a living- Why would you not want to use someone in the know? Especially with such a large purchase? If you are planning on relying on sites like Zillow, I have some bad news, they are not credible sources. Often the information they provide is not even accurate, so you shouldn’t depend on these sites to do your research for you.
Agents have access to rental rates and all the property history, including inspection reports, etc. so you can make an educated decision.
If you plan to sell your home in the future, or want to know what the neighborhood homes are selling for, your realtor can pull the comps or give you a CMA- comparative market analysis which shows sold and active comparables to give a good idea of what is the average selling price in your area.