I'm sure people have different reasons for layering libraries. I like to do it a lot, and while there isn't anything "wrong" with a single library, to me layering is still is a more deliberate practice than just "playing around". 😊
My personal take on layering - in addition to what Beat Kaufmann already explained:
What I like about it is the variety of performance. No matter how advanced and high quality our libraries are, we're still dealing with samples, and that means that even if we go great lengths to adjust velocity curves, volume, tuning and performance aspects, we are still hearing lots of same short recordings out of a limited pool being shuffled around.
We could say that layering expands this pool. By having the same part being played by two libraries simultaneously, one introduces more randomness, a higher number of permutations and different pairings of recurring samples, which results in a somewhat more lively, less predictable and hopefully a musically more attractive performance. Factor in VI Pro and its humanization features working their magic on both libraries as well.
Passages of short notes benefit from this by gaining more subtle variations in timing and tightness, resulting in a less rigidly quantized, more human sounding performance and less artificial "machine gun" effect. Sustained notes can sound more animated due to the subtle differences in tuning (no two musicians can ever play anything truly 100% in tune to each other), envelope and vibrato.
Another reason to layer libraries is in order to combine their unique aspects for a special kind of sound they couldn't quite produce on their own. For example, I like to layer the Appassionata Strings staccato with the one from Orchestral Strings. The Appassionatas sound very huge and warm, wide, almost chorus-y, and somewhat more distant than the Orchestrals - which in turn have a much more snappier, crisp, tight attack to them, a lot less "flubby" than Appassionata, most probably due to the smaller section size. I also like their firm, pronounced "body" sound.
So, if I want the size, warmth and grandeur of the Appassionatas, but also the aggressive, tight snap of the Orchestrals at the same time, what do I do? I simply use both and find a delicate volume balance between the two patches that gives me the sound I have in mind. It's a kind of a "cheat nature" approach to create something more unique and pleasing sounding.