If you have a Linux host connected to the Internet and..
if you have internal computers running TCP/IP connected that are connected to this Linux box via on a network, and/or
if your Linux host has more than one modem and acts as a PPP or SLIP server connected to other computers, and these machines do not have official or public assigned IP addresses (i.e. addressed with private TCP/IP numbers).
If you want those OTHER machines to communicate to the Internet without spending extra money to acquire additional Public / Official TCP/IP addresses from your ISP, then you should either configure Linux to be a router or purchase an external router.