doing VOD in the theatrical window allows two things important to smaller films
1) it allows a higher price point
2) it gives you better positioning
One of the big problems with VOD is the interface and search functions aren't very good, especially compared to itunes and netflix. Its really easy to get lost in the crowd. Getting better positioning increases revenue.
So you basically need to be in the VOD/theatrical collapse space if you have small films with no stars otherwise, people won't find your movies.
Problem is, a lot of theatres HATE that model. They see it as cannibalization (i think theres some truth in that) so its a direct competitor, therefore they dont want to support that model. Asking for some sort of rev share is a way of compromise. "I'll play your movie, if you give me a cut of the VOD".
Doesn't work on my side. If I can't get into the best theatres which will actively promote my movie, then its not worth it because I'm not going to gross very much revenue theatrically, while I will gross much more money on VOD. I see it as a pay to play arrangement where I'm not getting any upside from playing in smaller theatres since a lot of theatre's definition of "PR" is posters, trailers, print & calendar ads. If the Arclight in Hollywood wants to play my movie, then it probably makes sense. I'm not going to hold my breath that will happen.
It's not always that simple, but thats the basics.
Interestingly, the VOD platforms want to cut down on the saturation of this model so they are starting to require a specific number of theatres and markets a film is released in to qualify for this model. Since finding theatres that allow this model is challenging, it will cut down on the number of films using this model. This, thankfully, doesn't apply to me, since we have our own theatres.