Or makes for easier support...
DSL/Cable didn't support anything other then one PC connected to the modem for quiet some time.
They may not have
supported it, but it's always been possible by adding a NAT firewall/router behind the modem.
Thats actually what got me started with Linux. I had signed up with one of the first broadband providers in my hometown. Initially, I had asked for 3 IPs, one for my PC, one for the roommates PC, and one for the Windows server we had. IPs were cheep enough for 3, but when I bought a laptop, I didn't want to pay more for another IP, and worry about changing network configuration on the laptop all the time. I looked around, and found that commercial routers with NAT were still outside the consumer space and very pricy. So I looked into SuSE Linux, and eventually turned the Windows server into a Linux router and file server. I picked SuSE back then due to the boxed version including a decent manual and 6 CDs worth of software.
It wasn't until I moved to Austin a few years back that I switched over to using a commercial router.