A Subscription Is a Standing Order for Your Next Upload
Mechanically, subscribing changes routing. Your future uploads become eligible for the subscriber's home feed with a priority strangers' videos do not get, appear in their subscriptions tab unconditionally, and — for the minority who ring the bell — arrive as notifications. None of that touches the video they subscribed on; all of it touches every video after.
This is why subscriber count shapes launches. A new upload's first day is spent in front of the audience most likely to be offered it, and that audience is drawn heavily from subscribers. A channel with a real base opens every video to a filled room; a channel without one opens every video to whoever search and suggestion happen to send, which on a small channel is close to nobody.
The honest frame for a subscriber purchase follows: you are widening the room your next uploads open to, and raising the number the public reads. What happens inside the room is still down to the videos — a point this page returns to below, because it is where subscriber sellers usually start lying.