Because, typically, you are behind a NAT router, and you are after your WAN address, not the LAN address you were assigned by the router. Depending on the rout

Because, typically, you are behind a NAT router, and you are after your WAN address, not the LAN address you were assigned by the router. Depending on the router, it might be simpler and more reliable to interrogate the router. Also, some routers come with dynamic DNS updating built in (dynamic DNS is a common reason for needing this service).