Because a and v would be more typical variable names than rotateBy and around. Note that the author did not change the variable names in the translation process, they left them alone. So you could drop any variable in there but the calling of rotate becomes clearer (subjective) by switching to keywords and no longer relies on programmer discipline to have good variable names.

And yes, they probably could be used for other purposes. The names are mnemonics for "angle" (a) and "vector" (v). How you arrive at them and what you'd use them for could easily vary across a system.