Comment by andrepd

Then you don't understand the process at all. You will get something back, you will get an immense amount back. But (almost always) not directly. A mathematician will not suddenly solve a theorem that will enable a cure for cancer or better solar panels or whatever. But working in mathematics will build the gradual understanding that will enable those practical breakthroughs to take place. It's also the most important part of how the people that create those technical breakthroughs will be trained.