Any examples from the audience?

I know many of these might be characterized as engineering questions but a well placed concrete example helps many understand the math concept better.
What problems seem to satisfy students when they ask you ”What is it good for?”
I could not seem to think of a concrete example for series.
Please bring up any examples that strike you as I speak.

I know many of these might be characterized as engineering questions but a well placed concrete example helps many understand the math concept better.