If anyone is familiar with the cloud computing model, this is a similar take on it. Somebody has excess sever capacity (Gamestop has physical stores), rather than stand up your own data center, pay them to use that extra capacity give Gamestop a percentage off the top). Microsoft doesn't have to pay for all of that real estate to have a physical presence. Let GameStop pay rent, employees, all of those capital expenses. All of that surface hardware aside, I don't think they're taking much of a loss on the deal since GameStop only profits on a sale for Microsoft (I haven't looked at the deal in a few days, might be off on that). It will put the burden on GameStop to shift products if they want to make money on the deal. If they don't make a lot of sales, Microsoft doesn't really lose any money - yeah they didn't sell anything, but they also didn't pay the percentage, so no net loss.
It's not going to be a game changer in terms of sales, but it let's them have a physical storefront without needing to pay. I'd bet that GameStop was still seeing more foot traffic than the Microsoft stores. I got a friend still pre ordering shyt from GameStop for whatever reason