Like other guys have said, I use mental math for figuring TOD, sometimes Groundspeed based on change in DME and time, etc.
But I've also used it to figure out an excel file to determine how much money is saved, or lost, by tankering fuel. Nothing really hard, but some algebra was involved.
I think that math is important because it teaches you problem solving skills. I was never great in math, usually got Bs and Cs. I took Algebra, Geometry, Algebra II/Trig, and Pre-Calc in HS and then College Algebra and Calculus in college. Do I use all of that math every day? No. But I do use problem solving every day.
I had a bunch of classes in HS and college and said "I'll never use these as a pilot." Classes like management, economics, technical report writing, etc. Guess what? I became a Director of Safety and have used those classes in writing manuals and procedures, talking to company executives about the economics of safety, etc. Now I wish I had taken more classes like that and had taken those classes more seriously.