What is all that grading Fuss about?

Hey o7 im totally new to the Hobby and I have a now mostly finished Set of Surging Sparka (only Missing some SIR and IR cards).

All I hear people Talk about is grading. PSA 10, Black Label etc. etc.

From my current Point of View I cant really understand why people are so hyped about this. Im trying to get all the cards from the Set and I want them all to be in my Binder why would I want to have the cards put in plastic slabs so I cant really get them into my Binder anymore?

Can someone explain this to me?