In general, as the use case grows, every use case where the developers did not make explicit and careful provision for ensuring that order is not important, with quite non-trivial effort.

Even a lot of systems whose developers think they have no ordering dependencies are wrong in at least one subtle way without realizing it.

If you need to megascale, you're going to have to bite the bullet and build a system that can handle out-of-order, but there's a lot of systems out there where you don't need megascale, and you can get rid of that "quite non-trivial effort" to deal with out-of-orderness by asking for messages to arrive in order.

To get a sense of just how useful that can be... bear in mind that every time you open a TCP socket instead of a UDP one, you just made exactly that choice, to use an ordered message system when you didn't "need" one. Take a look at everything you do with a TCP socket and think about trying to run it over UDP, and not with something like QUIC that basically adds half of TCP back on it, but with UDP straight-up. That's what kind of things can use in-order delivery... lots of things.

Almost everything can be simplified by guaranteed in-order delivery. It's just that some things can't afford the downsides.