everything! wherever there is an input that affects an output, there's a function.

converting celsius to fahrenheit is a function: f(x) = 1.8x + 32

compound interest is a function: f(p, r, n, t) = p(1 + (r/n))^nt

gravity is a function: f(m1, m2, r) = G((m1*m2)/(r^2))

to find a correlation between two things, you can try to fit your data to a function.

computer programming is all about functions. even in object-oriented programming, objects are made of functions. and your entire program is a function of its input onto its output.