Another application is resource-constrained devices. I love the netbook form-factor, but my little Intel N200 machine buckles under the strain of running what amounts to six web browsers simultaneously (because everything is Electron now) in order to receive notifications from all the chat networks I have people on.

It can also be nice to have a kind of buffer layer in between you and the chat network, which doesn't necessarily have your interests in mind. For example, Facebook Messenger's Android app somehow managed to wake up my phone's screen every time it received a notification, despite my turning off every related setting and permission I could find. So I put it behind Beeper and the problem is gone.