My understanding is homomorphic encryption is extremely useful if you want to do computation on someone else's server with data that you don't want to reveal.

My understanding is homomorphic encryption is extremely useful if you want to do computation on someone else's server with data that you don't want to reveal. However, in practice, you reveal some information about the data if you do computations in it. One notional example is if you see a bunch of XORs in data. You may not know what the data is, but if you see a bunch of XORs, you can make an educated guess that some sort of cipher operation occurs (as XOR is used heavily in it).