It's mostly additional capacity for your Google cloud storage. It also includes a VPN and a couple of premium Google Photos features like magic eraser.
They've actually been ramping up efforts to get people to pay for storage lately. I've always been fairly close to full, but lately they've started popping up brightly-colored warnings on various interfaces telling me to buy more space.
I did the opposite and got down to like 80% full, and the warnings are still there!
This is a far cry from the original statement that I'd never run out of email storage. (Yes, I use it for other things, too, but I've found myself having to delete emails with attachments to keep storage requirements down.