Example: Being able to test function equality makes it easier to do certain forms of caching which makes it easier to build more performant web pages. Elm’s built-in support for lazy HTML is an example of such a thing but it isn’t the only such thing.
More broadly for better web pages, function equality crashes slip too easily into production and it would be nice to either see them supported or prohibited by the compiler. (See message 5 in this thread.) Prohibition by the compiler would be pretty disruptive to existing code since any type parameters with values tested for equality would presumably need to be revised along the lines of comparable. So, I was looking for an implementation of function equality that maintained referential transparency.