You can sell access to your on demand videos in a few ways:
Collections show your customers a list of videos to rent (purchased individually and can be watched in any order)
Courses show your customers a list of videos that must be purchased all at once and can only be consumed in one order.
On-demand subscriptions allow you to include access to both collections and courses, allowing for one purchase to be made and access to be granted instantly (instead of a la carte).
Collections are ideal if you'd like you're customers to be able to pick and choose from all videos within a collection, in any order they choose. Collections also have a 'scheduled go live' or 'drop' date feature, where you can schedule video debuts.
Courses allow you extra control over the order in which the videos are watched, require a little extra setup to make this possible.
In summary, creating an on-demand subscription isn't required but allows your customers to have an easy one time purchase that continues to give them access to the things they want to view.