Typing in your friends name (or asking them for their username if it's non-obvious) is significantly easier than exchanging bank account details. Especially when you're dealing with groups of people.

Here's my user flow. I sign up and connect w/ Facebook. I connect my bank account. Separately, my friend who I rarely see does the same thing w/ his Facebook account and bank account. A few months later we meet, I pay the full check and he Venmo's me 20$ for the meal w/ a few taps on his phone. He doesn't need to ask me anything. Because we're already Facebook friends, he has access to send me money.

I have Bank of America. For the same transaction to happen I would need my friend to sign into his bank account on a mobile phone, initiate a wire transfer from his checking account to my checking account, which requires me to provide him my checking account number and zipcode and full name.

And that's just a 1 on 1 transaction.

The most common use case is if I'm eating in a group of say... 6 people. Instead of splitting the bill 6 ways which many restaurants won't allow one person will front the bill. The rest of the group just "Venmo's" the person who fronted the bill whatever they owe. No exchange of account numbers, no asking for Paypal email address, none of that. The assumption is that everyone has Venmo and everyone's connected on Facebook. As long as those 2 facts are true (which is common for American millennials) the convenience is bar none.