This had 3 uses
- Allowing Karma to push lanes/towers more efficiently
- Allow for a better aiming of your Aoe shield
Allow for a better aiming of your AoE damage
The second point is not that great actually, just a nuance. Sometimes using a minion allows you to shield/speed 4 allies instead of just 2. Since the removal of the shieldbomb the most important reason to keep it was nullified. It was awesome to use even things like jarvan's flag to surprise others with the RE burst.
Whether Karma actually needs this form of pushing/annoying laners, or whether this power can be put somewhere else instead is debatable. There is also a discussion whether it would be better to allow minion cast ONLY when using it in combination with mantra. Personally I think this would make it very unintuitive for people learning Karma, even though mechanically it could make sense.
But this is just my 2 cents.