At a personal level .. buzz droids were one of the low points of the prequels. Why use droids like that when a simple missile would have been more effective?
Firstly, terror weapon. Secondly, sabotage droids can target specific systems and probably have specific uses for capturing ships or disabling larger vessels, and if you have a munitions shortage but a lot of sabotage droids you'll use them where you can. Thirdly, their deployment method is via cloud rather than direct impact, meaning you don't have to hit the ship. Given an X-wing can fly through an exploding TIE fighter and come out mostly unscathed a simple shrapnel cloud isn't going to hurt a starfighter much.
Why didn't Obi Wan and Anakin just use the force to remove them? That whole scene just seemed so gimicky.
Same reason you don't pull out ticks.