The vast majority of research-level pure mathematics is never going to get used in science or engineering. Obviously it is hard to predict what will be useful, but for the type of mathematics that is unlikely to be, there is a question as to why we care about it, and that almost has to come down to beauty in some sense - some mathematics gives us a new lens to look at parts of the mathematical world and others chip away at problems in more mundane ways in the hope that they inspire or contribute to new parts of mathematics that are beautiful.