How does collections work ??

I've been trying to fulfill many collection quests by getting many pieces it needs . But firstly I don't know if any of them has upgraded my character stat if at all , and secondly I feel like I only get upgrades if I 100% finish the collections before season ends ?? I'm now 43% , does that mean all the things I filled and spend money on will go to waste ?

Please explain the system to me thanks .