Well, the why is to commemorate the release of Dragon Quest Heroes, the new PS4 and PS3 Hyrule Warriors-like crossover that will combine the world of Square Enix's RPG series with action crafted by Dynasty Warriors devs. That's due in spring 2015. As for what that thing is - the happy little slimy dude - it's a removable USB cover rather than a permanent fixture. We're filing it under S for "Sure..." and Y for "Why not, I guess?"
The Metal Slime PS4 wasn't the only new thing Sony showed off yesterday, because that would be super weird. For the lowdown on all the major announcements from yesterday's presser, check out our round-up of the highlights.