Could it be the difference between a built-in dial-up modem and a built-in broadband (Ethernet, RJ-45) conector? Unlikely, since you also talk about a modem.
Maybe for fax?
What does the user manual of your older Dell say about this port that you plugged that cable in?
I have just replaced my older DELL desktop (running Windows XP) with a 2nd hand HP 8300 Elite SFF, (running Windows 7). Trouble is, the DELL had a phone line cable that plugged into a port in the back. This cable ran from the phone jack in the wall through a line filter then into the back of the Dell. The line filter itself has a separate cable that runs up to my modem.
My problem is, the replacement HP does not have a port to accept this phone cable, only a similar but larger port for the modem cable. What is the purpose of connecting the PC to the phone line and why would the HP not have this port. Is it even necessary?
Hoping someone out there can help as I am ignorant of such things! Thanks.