This helps you in two ways.

First, you compare the cars first hand to make sure you get one that you like. Sometimes brands change the models quite a bit over the years so even if you liked a Honda Accord for example from years past, you might not like the interior or the feel of the new one when driving. So don't assume you know what you want based on the past.

Second, if you take multiple test drives, the dealer becomes familiar with you but knows you are shopping around. So that loosens them up to try and finalize a sale with you. If you also do this towards the end of the month when they are trying to hit quotas or win a sales competition then that is extra bargaining power for you.

But most of all remember that this is your money and you have all the power as the buyer. Dealers know that you can easily go to another dealership at any time. And with car inventory prices easily available online, there is no excuse to be unaware of fair value for any make, model, or year.