Can somebody explain in a sentence or two what this actually is and what the benefits are?
The main USP: you own it, you're not renting it. You're not beholden to big cloud's pricing strategies and gotchas.
Secondary USP, why you buy this rather than DIY / rack computer vendor: it's a vertically integrated experience, it's preassembled, it's plug and play, compatibility is sorted out, there's no weird closed third party dependencies. Basically, the Apple experience rather than the PC experience.