It could also let people experience something "first-hand" that they'd never experience otherwise.
(For example, I know I'll most likely never clear an ultimate because I just don't have the time for that commitment. But being able to stand in one? That sounds pretty cool.)

So being able to enter via spectator mode could further boost people's interest in the mode itself and in different pieces of content.
(And who knows, perhaps it does inspire somebody, who was previously not gonna try, to give it a go.)