If you're seriously playing a game online then you own the game and the same goes for everyone else that's serious about it. This is some extra casual stuff. What kinda skills is a person that doesn't own the game yall playing really bringing to the table? Maybe your friend gets to try a game out but I fail to see how this is some game changing feature.
This is essentially a virtual version of letting your friend play the game with you at your house except it's over the internet. Back then brehs came to the crib and played a game real quick or played with you and sometimes they never planned on copping the game. My boy like that right now. He had a PS4 and never copped 2k14 but would always come over and play my bro and others in it pretty much daily.
Nowadays demos aren't that common for new games and with the heavy influx of new titles being released weekly, having the ability to instantly try out a game can increase sales of a title through word of mouth.
There are other times where people simply don't have the money to burn on a new game right away. Access to play a game instantly without going out and paying for a rental is very convenient. A friend of mine bought a PS Vita and a copy of that ps all stars battle royale game simply because she had access to a beta that I shared with her.
Now imagine that same scenario where there isn't a beta for the game and they just want to play it real quick to decide if they're going to get it. It beats coughing up $60 to test out a game by a country mile. Not everyone is brimming with money to spend on brand new games or even used games.