Many network utilities (ping, ssh, traceroute, etc) will take an interface name to signify the source address, and then query the interface for its list of addresses and use the first of the appropriate address family.

This is handy when you want to source packets from an interface with a dynamic address (such as an IPsec/GRE tunnel or a DHCP'd subnet) but don't want to be editing the configuration file on-the-fly.