One small correction. It should not really be "posts can reward other platforms". The proposal allows anyone to be rewarded out of a posts payouts.

A potential use case for this is to reward a platform like busy or esteem for posts where you use that platform to compose and post it. This could be built right into the busy submit new story. This use case is discussed at length in the proposal, though i really don't think it will be the primary or even a significant use for the feature.

But you can also do this for commenters on your post. For people who have supported you in the past or just anyone for the hell of it.

Personally, i don't think the platform based thing will gain much traction (because why pay, say busy, a non-trivial part of my post rewards when i can just compose the post on busy and c/p it on steemit to post it.) But i think giving to other people will.