Yes. I did some challenges but ended up just going for Main Quest progression. Postgame opens up new stuff, altered environments, you can unlock a couple things like Battle Challenge and Boss Rush and since you have all of your abilities barring ones you need to unlock in the skill tree you can start exploring everything Layer by Layer.
I completed the game at 245 Bananas and the total amount is grand total of 777 Bananas
And of course there are all the Music Records and Outfits to unlock.
As for the purpose of Postgame I'll let you figure that one out. I think Postgame will be harder then main game considering it only took me around 8-12 hours(maybe a bit longer?) to get to where I am now.
Very addicting game.
It's much better then Odyssey or the 2 Zelda's due to traversal and the freedom it allows you. Not having to worry about Stamina helps a bit too. And this is all stuff you the player can do on your own and not stuff like Autobuild from TotK.
Probably the closest we will get to a DK64 2. And other then jumping down Chasms being a very clever loading screen or sleeping most of the game is very seamless.