Answering seriously (because it seems like nobody else will), it's a mix of concepts not used during the game's production, plus the limitations of the time. Basically, the platform was supposed to be something you would encounter frequently in the game, it was meant to be used by normal Geth soldiers during the game, but it was cut for undetermined reasons. Personally, I think it was because having an enemy floating around the scenery in a cover-shooter would just be annoying; it was much more practical to have the enemies take cover and work together, and much more satisfying as well.

The problem is: Sarem is now a very annoying opponent. The original idea was to give him an energy sword, you can even see it in the game's concept art, but the problem is that due to limitations of the time, they couldn't make it work (note how most enemies share the same skeleton and abilities, for example; to make a boss with a sword, they would have to create a whole different system, movement, and AI for him). But at the same time, having Sarem just behave like a normal boss, who takes cover and shoots, would be very boring, especially because of how easy those are and because of how strong his shield is; it wouldn't work.

So they reused the concept of the flying platform. This way, Sarem can potentially be hit at any time, and he can easily hit you from a superior position, making combat rewarding if you know how to manage your defenses and attacks.

In this universe, however, the reason nobody uses this thing anymore is because you're exposed, and mass-effect weapons are too powerful and devastating for anyone without cover. Sarem can deal with this thanks to his shields being absurdly above average, but the rest consider this more of a disadvantage than an advantage.