What is the difference between XRL main outs and LINE outs on a mixing desk? Don't they both output the same Line level signal? What is the ideal use of one instead of the other? I still get confused about this.
The biggest and most relevant difference is that one uses an XLR connector and the other uses a 1/4” TRS connector.
This is helpful because you might want to plug the outputs into something that has either 1/4” or XLR inputs, but not both, and you don’t have and can’t easily obtain an adapter.
Another reason to support both standards is for different use cases where again you’d rather not have to need an adapter cable. For example, you might want to connect to a TRS patch bay with normals, which you cannot do with an XLR connector (and an XLR patch bay can’t have normals). Or, you might want to be able to daisy-chain cables to reach an arbitrary or unexpected distance away, which you can easily do with XLR cables but can’t with 1/4" cables without several F-F gender changers.
I can speak from experience that having to have XLR-1/4" adapter cables in addition to your stock of XLR and 1/4" cables is quite inconvenient.