Its March 14th which means it is international Pi Day! I use pi pretty much every day doing signals processing. Sine and Cosine rely greatly on pi. Angular frequency is found by multiplying your frequency by two pi! Pi is used to bound what frequencies you can represent with a digital signal given a sample frequency. Pi is a fundamental aspect of Euler's formula (e^(i*pi) + 1 = 0) which gives us access to an entirely different way to solve equations. It is a wonderful number and enables a huge aspect of mathematics. I wouldn't be where I am today without pi.