Web servers frequently host multiple domains or applications on a single physical IP address using Virtual Hosts. If a developer sets up an internal application without public DNS tracking, vhost mode will find it.
The --domain flag specifies the base domain name to append to each word in the wordlist. You must use the target server's (not a domain name) for the -u flag. gobuster commands upd
/usr/share/wordlists/dirb/common.txt or /usr/share/wordlists/dirb/big.txt Web servers frequently host multiple domains or applications