The "Documentation" section actually shows you what it does (better than the FAQ explanation some have mentioned)

https://www.mailvelope.com/en/help

It's free for personal use, but there's also some paid tiers. As far I can tell it looks like the only thing Mailvelope does that is outside of the extension/your-local-machine is host their own key server: https://www.mailvelope.com/en/faq#key_server

Unless there's something also it does that I'm missing?

FYI, it's open source: https://github.com/mailvelope/mailvelope