In the dark ages, before always on high-speed broadband internet connections, people used devices like these to reach out to other machines. Of course in 1989, the year of this ad, there was no Internet to speak of. At least not one that was publicly accessible for most people. However, there were still ways to communicate with the digital world.
From the early 1980s through the early 1990s using dial-up via a modem like the one pictured above to call a local BBS was the way to go about it if you had the right equipment. While you were only connecting to a single machine with your modem, a BBS would provide files to download (freeware/shareware games, apps, etc.), games to play (usually turn based and against other players who would take their turn when they dialed in) and message areas. Many BBSes made use of FidoNet which was a networked messaging system like Usenet in many ways. BBSes that were part of FidoNet would exchange messages and thereby allow you to communicate with people all over the world.
Modems progressed through various speeds throughout the years. In the BBS era these speeds progressed from as low as 300bps (bits per second) to as high as 56kbps. However, one of the longest lasting speeds was 2400bps which is what the SupraModem 2400 provided. Modems providing these speeds were introduced in the late 1980s and were common through the early 1990s. Though faster modems were becoming available at reasonable prices, my first modem was a second hand 2400bps modem for my 486 based computer in 1993. By 1994 I had a 14.4kbps modem which was pretty new at the time but price wars were rapidly driving down modem costs by this point. 9600bps modems were available earlier but they were expensive and by the time the protocol was standardized, work was nearly complete on a standard for 14.4kbps modems.
Supra was known for producing decent modems at a good price. An external model like the one pictured above would work with just about any computer available at the time with the right interface cable (Amiga, Commodore 64, PC, Apple II, Atari 8-bit, Atari ST, TRS-80, etc.) and with the right software you could communicate with a BBS running on any type of hardware. I had an internal modem for my 486 based PC that fit into one of the ISA slots because it was cheaper but external modems offered more flexibility (e.g. I couldn’t use my internal modem with anything other than a PC).
The above ad is from the October 1989 issue of Compute!’s Amiga Resource.