because there are things that he/she can spot that can impact your decision on purchasing the house, it can affect the negotiations on the price. There are things that while readily apparent to him/her won't be to you, but that doesn't mean they catch everything. Ours filled out a binder full of things that he spotted when we were negotiating. Some needed fixing, some were recommended fixed. And after we moved in, there were things that he missed in those 2-3 hours he spent inspecting.
It doesn't have to be either pointless or foolproof. It's somewhere in between, in terms of efficacy and liability.