Everything works in steam (exept during the holiday periods) - send invite to friend - play.
Easy to backup game library, move to another drive etc.
Easy to purchase and gift games, add to wishlists etc.
Easy to change download locations, set download limits, set priorities, turn off downloads during gametime, override limits, pause updates on specific games etc..
Lots of details about games, including whether they are co-op etc.
Web version is also available.
Store has a decent layout, review system is easy to see at a glance, search has heaps of options and works well, browsing games is quick.
Lots of community forums, discussions and mods.
Try doing that on Ubishit, Origin, or the beyond terrible MS store (no really, what a steaming piece of sh*t).
I haven't purchased anything on GOG or Epic's store as i've never had the need. However, i had a quick look at Epic's store yesterday to see the price of Ashen and the details were extremely lacking.
In an ideal world all games would be available on all stores with the only difference being the price, and i'd happily pay more to play games using a store that's doesn't waste my time, and that actually works.
/rant.