Just because you don't need money, it doesn't mean you cannot use it. Hopefully they will be able to use that money to speed up their revenue growth and quickly pay back stripe and be left at a higher revenue point than they started. But just because you are in a position where money helps you (pretty much everyone?) it doesn't mean you "need" the money as in you are searching for loans or investments or have some desperate need right here and now.

Edit to add: It's not a loan, they aren't accruing interest and the cost is fixed. This would be a weird type of deal if not for the 5% revenue fixed at paying back the advance and fixed cost. If not for that, they would essentially be getting a better and better loan the furter they waited it out, as the effective annual rate goes down as time passes. It's still true that the deal is better the longer you take to pay it back though, but the revenue payback means that you have incentive to pay it back fast, because that means you are making money. It's an interesting deal.