Can someone explain Edge/Shadow/Bartz USB and why they're important?

I consistently see these 3 relics hyped up around the sub, and I've been saving like mad for them.

And then I realized I have no Idea why I'm saving for them

I read their effects, but could someone explain the combos/appeal of these USBs? I want to know which to prioritize. I have Bartz OSB/BSB1/Enwind and Shadow SSB, if that helps.