"Back in the day" these were popular because you might have a modem, but not IP internet access. So places that provided a dial up Unix shell had value. That is, a plain terminal dial up. Dial up to their shell, ftp a copy of some software, then download via zmodem or Kermit.

http://nyx.net was similar.

After slip and ppp became popular some still used these as a home base of sorts. Like you might keep files there so you could get to them from different client machines when traveling. There was also typically a community there that would help with questions, share news, etc, not too unlike HN.