They’re fairly useful in applications that use numeric IDs. For example, if I’m using SQL, and I have a table with an AUTOINCREMENT primary key, I’m going to have a lot of numeric IDs. If I want to reference these in a config file of some kind, I don’t want to have to read them as strings and handle the parsing on my end.

Even if you’re of the opinion that IDs shouldn’t be numeric, there are a lot of cases where you’re stuck with integers—on Linux, user IDs, group IDs, and inodes are just a few examples.