I know the definition of a category and know several examples. But I have not studied the theory of categories.   Much of mathematics I studied can be written as a category and a functor.  But for me, a category is just an organizational language. What is the merit of studying category theory?  What can I obtain by reading Mac Lane's book?  Is it necessary to learn category theory to be a good mathematician, especially for number theorists, algebraists, and topologists?
Okay, so I understand what calculus, linear algebra, combinatorics and even topology try to answer (update: this is not the case in hindsight), but why invent category theory? In Wikipedia it says it is to formalize. As far as I can tell, it sort of generalizes a bunch of fields in mathematics, like in topology, graphs and groups we have isomorphisms and automorphisms and homomorphisms, it seems to me like it just finds a bunch of results that are true to these functions in a bunch of disciplines. But why would generalizing that be of use? I mean, wouldn't it be easier to prove them specifically in each of the important disciplines?
In .NET, what's the best way to prevent multiple instances of an app from running at the same time? And if there's no "best" technique, what are some of the caveats to consider with each solution?