From the repo the project creator linked in the README (https://github.com/yrutschle/sslh): "A typical use case is to allow serving several services on port 443 (e.g. to connect to SSH from inside a corporate firewall, which almost never block port 443) while still serving HTTPS on that port."

MIT's SSH server pool (athena.dialup.mit.edu) listens on port 443 for both web-based login using a JS terminal and for actual SSH, using sslh as the multiplexer. SSH on port 443 is useful for people who are behind weird guest that only allow certain protocols out.

a different usecase / protocol selection

ZNC uses the same port for HTTPS (webadmin) and ssl for irc.

Sure, those can be different ports, but if you don't need to run them on different ports, why waste the extra port?