So that I don't have to retype them every single time. e.g. I'll only need to type "Fourier transform" to get the template.
Sure, I could look up the Latex code from the page source in Wikipedia, but it would be nicer if I could search the code directly.
You can easily use $\backslash$newcommand to create your own package with commands you often use. Another nice website to find symbols, whose name you always forget, is http://detexify.kirelabs.org/classify.html