I still have no idea what capers are used for. I see them in delicatessens all the time, but I don't buy them because I have no idea what I'm supposed to do with them.
They're a little pop of salt and acid, so go really well with, eg, oily fish. I make a warm smoked mackeral salad which would be a bit meh without the capers.
They're great with smoked salmon. Put a little smoked salmon, a squeeze of lemon, a few capers, and maybe a sprinkle of fresh dill on a crostini or cracker. They're also nice in pasta sauce.