With the SharePlay feature being limited to a max of 1 hour and you can't use your own PS4 while someone else is remote playing it, the best feature of SharePlay to me would be to let a friend try out a game to see if they're interested in buying it as a kind of demo. Or if you are in an annoying point in a game that you just can't beat, you can let a friend try to beat that area for you.
Sources: Kotaku & Gamespot
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