i think that every unit in the eldar codex is made to support the other units in the rest of the army. for example, eldar combat troops, specifically banshees are helped greatly by having striking scorpions supporting in the same combat. jetbikes are fast which is great against static armies, and can soften up or harrass dangerous units. theyre quite cheap for a bike mounted model and as most eldar are only t3, theyre much easier to kill. one or two less models can make the diference to expansive aspect warriors.
with their jump out, shoot and jump back they can stay away from the enemy to add survivability, and annoy your opp the same way tau battle suits do. theyre only fault IMO is the shit range of the shruiken catapult.