On several secondary applications, it asks you "why our school?" and open houses can be pretty good places to learn about specific things that school offers.
Otherwise, it's just a good way to get some feel for the place if you're deciding whether to apply or not. You can also talk to actual students, and possible an admissions person if you have specific questions etc...
I think they're pretty worthwhile if you have the time, but it doesn't actually help your chances of getting in as far as I know. I could be wrong though, since that last one I went to an open house, they took some of my personal info. So I imagine an adcom could look me up and see that I attended their open house. I really doubt it plays any major role though. It can't hurt to go, but it may help. Most likely, open houses will have no bearing on ones chances at a school.