It was far and away the simplest and easiest way to quickly transfer money electronically, at least around ~6 years ago. The "big players" that have payment apps either weren't launched or were just getting started, and PayPal just had/has this bizarre level of BS required for a process that should just take like 2 or 3 simple steps.
Nowadays I have friends that occasionally put amusing messages and stuff in their payments, but I don't really get the feeling that people are actually showing off much. Most of the payments I see are between SOs or roommates splitting utilities+rent and such. There are probably plenty of people who do enjoy this part of the app, but Venmo just seems like a great product regardless, so I don't know how key it actually is.