Modular arithmetic helps answer questions like: What is even+odd? What day of the week will it be 100 days from now? What computation do you need to do?

(rem(100, 2) = 2, so Tues+2=Thurs)

Source: Brynmor Chapman, Mathematics for Computer Science (MIT: OpenCourseWare). Licensed under CC BY-NC-SA 4.0.