[Twey]

Category theory is actually a ‘simplified’ graph theory, i.e. you can see categories as a restricted class of graphs. E.G. ‘Category Theory for Computing Science’ introduces categories this way (a category is a directed graph with associative composition and identity; the free category on a graph is the graph with all identities and compositions filled in). But the restrictions (associative composition and identity) are harmless and natural for programming applications where there's always a notion of ‘do nothing’ or ‘do one thing after another’, and unlock a lot of higher structure.

[sesm]

But what's the utility of this definition? Does it help solve or prove something?

[Twey]

It helps you build an intuition for categories, if you're used to graphs :)

If you have a working intuition for categories then in most cases the specific formulation you choose as a foundation doesn't matter, just as most mathematicians work nominally in set theory without worrying about the subtleties of ZFC.

[sesm]

IMO the right intuition about a tool comes from applying it in the context where it provides a real leverage. In case of Category Theory that would be advanced algebraic topology (not re-phrasing basic things which are easier to understand without CT).

[Twey]

Rephrasing things is useful when it allows you to draw equivalences with other things that might have results or insights that translate across, which is something a lot of people use CT for. As with most maths the payoffs aren't always immediate, though, which leads to a lot of frustration with from programmers who expect immediate results from their insights.

From a programming perspective, though, a fair few important things have (by now) turned out to be useful that have come as a result of rephrasing programming in mathematical terms. Some top contenders are monads (directly from CT — think Haskell, but also LINQ, JavaScript Thenables, async/await, Rust Option/Result combinators, parser combinators), proof assistants and modern approaches to type systems, linear/substructural logic (think Rust or C++ move semantics), functional programming (which is now embraced at least to some extent by all mainstream programming languages), functional reactive programming (the core ideas behind things like React), et cetera.