There's hundreds of little things that a surveyor checks, some of which are farily obvious, some which are fairly obvious once you're familiar with boats, and some which aren't so obvious, and some which you'd never think of checking but the surveyor would. For example, when we had our boat surveyed he picked up on the fact that the gas locker floor was needing attention and the cutlass bearing was worn - the first you might think of checking, the 2nd would require the boat to be out the water and you'd probably miss yourself. Since the gas locker floor was pretty much level with the waterline, it required dry docking again, so it ended up costing a lot of money - which we didn't pay for.
If the cost of the faults picked up (which the seller agrees to have done or discounts the cost of the boat by) exceeds the cost of the survey then you have made money by having a survey and subsequently buying that boat. If the costs of the faults picked up don't exceed the survey cost, then its probably a very good boat anyway and you can be reassured that.
Hopefully others can cite practical examples of unusual faults which a surveyor can/has picked up, but which you'd never think of checking yourself, partly because you've fallen for the boat and are starting to become emotionally attached to it.
Also in our case, due to the age of the boat, our chosen insurers required a survey and valuation - so it needed to be done anyway.