You're right that it's essentially arbitrary.
I know some globally touring producers who just put reverb as an insert on each channel they want to have reverb, changing the parameters for each - and it still sounds good. I know people who have a setup similar to what you're talking about - one short reverb and one long reverb, and you send your instruments to one or the other (though you wouldn't necessarily want to have one instrument sent to two reverbs at the same time just for the sake of clarity, but that doesn't mean you're forbidden from doing it and it might sound good!) and change where you're sending when you want things to have the SUPER LONG REVERB TAIL. As long as it sounds good and not just different, you can do whatever you want!
And with regards to pre-farder sends, that's an option most DAWS have as either a global option or something that is enabled on an individual send. For example, in Ableton you would go to the bottom half of the master channel in Session view and click the icon that says "Post" to change them to "Pre", which affects anything sent to that return channel. In Logic, you'd just route your track to whatever bus has your reverb on it, then click and hold on the send and select "Pre Fader", which would only affect that one channel and nothing else sent to that bus.
As far as why would that be important? One example would be for the reason you mentioned in the OP - when we hear more of a reverbed signal than we do of the source sound, we perceive that sound as being far away and "washed in reverb". Instead of a vocal sounding like it's right in front of you and you're inside a giant stadium, you can have it sound like the vocalist is shouting from the stands. Another good example comes to us from the world of moving pictures - if you had a person wearing hard-heeled or high-heeled shoes walking off camera down some long hallway with tile flooring, you could lower the volume of the channel without lowering the volume of the send to have the footsteps fade off into this very reverberant place where the echoes would continue well after the source sound was far out of sight.