Why do I need it?

Who doesn't hate waiting a long time for the cobblestone to be smelted? With this datapack/mod you're allowed to finally put cobblestone into the blast furnace and wait a bit shorter. It also adds things that you couldn't smelt/blast at all like the raw iron block turning into a normal iron block.

What about my XP?

You don't have to worry about that. If you smelt/blast an raw iron block you will get the same amount of XP as if you would've smelted/blasted 9 raw iron