One big thing they’re used for is sort of multiplexing port 80/443. You have one daemon listening on them, and you can have multiple domains pointing at the same IP. The reverse proxy will figure out which backend service to forward requests too.
Proxies like Caddy and I think Traefik also automatically manage SSL certificates. In many cases you could have your application server handle SSL, but usually it’s a good idea to have dedicated software for this.