Context: Someone is trying to purchase an expensive product they not only don't need but won't have much use for. The result they are expecting to get can be achieved in a much simpler way without the involvement of said product, but they insist on getting it anyway. Think of a not-exactly-technology-savvy-grandma eager to buy a laptop to write down her shopping list.
In my native language, I would say they're buying roller skates for a fish. Is there an English equivalent?