Como dizer "Pra que serve isso?" em inglês

I was talking to my teacher about how is difficult to express what we think in English sometimes once our way to think is different and I told her that a day I was trying to know how to ask: "Pra que serve isso?" So we stayed a long time trying to find a good way to ask that.

I though I could ask: "How does it work?" but does not express exactly what this question does, I don't want to know how it work, but its useful. Would be right to ask: "What is it useful for?"