In a nutshell, it goes like this:
Publisher: Will you stock our new game?
Retailer: Only if it's at least the same price via Digital Download services. Otherwise, no.
Publisher: I'm not sure that's what we want. Digital Editions cost us less, and we'd sell more at a lower price point.
Retailer: We'll put it another way. If it is cheaper on Steam, we'll remove your games from our shelves. All of them. Across all platforms.
Publisher: Now that you put it that way, I guess an RRP of £39.99 for the digital edition is reasonable.
Retailer: Good. I'm glad we came to an understanding.
I'm not sure this scenario carries over to big companies like Sega. The story comes from some smaller developers who ran in to the situation a few years ago. They now release solely on Steam, GMG etc., but they are PC only anyway.