An out-of-the-box solution to self-hosting a wide variety of services without much setup.
Can be used to implement IRC, Matrix, XMPP and E-Mail hosting for a circle of friends (also takes care of simple account management afaik); some web office software (collaborative editing, wiki, website hosting) file sharing (I2p, torrent, bepasty, plain file sharing) and privacy uses (metasearch engines, proxying, i2p).

Everything seems to be documented pretty nicely on the debian wiki. Probably less flexible than docker container setup but not bad.