no its not for that as most calipers have a bolt or clip that holds the pads in place anyway.
it is for when you remove the wheel and put the bike in your car ect so if you knock the brake lever the pads dont get pressed together and thus you need to remove the pads and rest the pistons to get the wheel back on.
Some pads are not held securely in place, when the disk is not present, and delivery people throw the boxes about, and some brakes a pad can fall out, or maybe even the lever gets operated.
Apparently, the plastic piece prevents pad movement, and my brakes are not even hydraulic.....so its not just for hydraulic brakes as you imply!
That was from my really good friendly LBS here in Germany, after I took it in to show them!
I did not have a clue till then.....