My guess (and it's only a guess) is that with Venmo, the girl scout organization gets the money right now, instead of having to wait for the little girls to send it in . . . whenever.
Another guess, especially these days, might be safety. Though I'm sure it also cuts down on boomer sales. I also don't have venmo. I remember when I first got Steve, I signed him up for dog school and the girl would only take electronic payments. We ended up on a big back and forth before she agreed to use paypal, which was the only epayment I had, and that she thought was some kind of boomer thing.
Personally, and I know much of it is my paranoia, I hate all these epayment systems that you have to link your bank account to, at least for individual transaction stuff. Of course I use epayments for the electric bill, etc. I don't even use my debit card for anything other than getting cash from the ATM.
I supposed a good unintended consequence of so many people no longer carrying cash, is that I run into a lot fewer bums on the street trying to hit me up for money.