It is for cooling the "oil" in the outdrive. The oil is the outdrive to protect and lubricate. Bravo III's get SUPER HOT - and drive showers are MOST important if you have a Bravo III or if you have a super fast boat with a bravo 1 or 2. Bravo III's run SO HOT because the big set of gears is up on the top end of the drive. When you are under way, that part of the drive is 100% out of the water and the spinning gears and gearcase gets very hot.
The drive showers pick up the water and redirect it to the top of the drives while under way, thus keeping things cool.
Nothing is going to break tomorrow if you don't have them, just long term insurance...but great cheap insurance for sure.
Found some specs on drive temps - without showers.
Drives listed in order by operating temperatures:
(hottest)
Bravo III (270+ degrees)
Alpha (all) 265+ degrees
Bravo II
Bravo HP, XR and ZR Drives (due to their performance applications)
Bravo I
(coolest)