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.

Because those are names the caller decided upon, so they can just as easily misinform the reader as correctly educate them. The caller could have mistyped the call as

     t.rotate(around, rotateBy)

and the error in the effect might seem very mysterious indeed. The beauty of keyword args is that the mapping of caller's name to function argument name is explicit, and therefore reads correctly or incorrectly immediately at the callsite

     t rotateBy: point around:degrees

is obviously an error, without sending the reader to look up the definition.

Compiler-enforced clarity rather than relying on every individual piece of client code to always name things well & never make a mistake.