Using the 4K panel, Steam, as it is now, is a postage stamp. Any change that can look better with low resolution screens as well as high resolution ones makes perfect sense.
Valve has long been sluggish with change so this new design is a move in the right direction. Still there are many changes that would make this better but at present there is no forum discussion yet.
The new grouping is designed to better show the genre and background of the games in a players library. Friends activity is also show but given I play one game a lot, the team is pretty much focused.
The new design is intended to be able to bring gamers closer to the action. For example some games may be having a tournament which many may be aware of.
Valve also wants to be sure that players are not overwhelmed with crap which would take away from the experience.
The new UI is now in open beta and it probably will be released generally in a month or two.