What does this bring compared to apt-get install-ing an established mail server like procmail or exim and configuring it to work as a relay for the local LAN?

Offering a UI to quickly set it up.

Reading through exim docs has taken hours of my life and it still caused weird issues. Using software that works for setting up a multinational email server to make your homelab send messages to Gmail is a massive time sink and kind of ridiculous really.

I haven't tested the code myself, but I'd prefer a simple system with limitations over a complex system that can be configured for simple tasks.

For this very simple relay, there are lightweight alternative like msmtp.

https://packages.debian.org/trixie/msmtp

I think dma (https://github.com/corecode/dma) would work for this purpose and has a much easier config than exim.

setup wizard and web UI