I think it varies by the person. Some people already have their minds set on a school beforehand so for them it's pretty much just a vacation. For others who are split between a few schools it can heavily impact their decision.
It's like interview day but people say they more relaxed and pay more attention to the tours since they aren't stressed out about the interviews. You usually get to see more of the facilities and maybe tour some of the clinical sites. You also might get to talk with current students and faculty as well as your future potential classmates and see if your personalities mesh well together. If you're already set on a school and want to save some cash I don't think it would be worth it (assuming you need to travel far and would require hotel arrangements). If you're really torn between a few schools though I think you owe it to yourself to check them out to make a more informed decision since you'll likely be paying tens of thousands of dollars to them a year anyway.