Does it help to have a dial-up modem on the computer? Can it be used for anything?
To explain to your grandchildren how we connected computers to the Internet in the 1990s and early 2000s, and to teach them that WiFi and mobile data have not always existed.
And to tell them that the first mobile phones appeared in the 1990s, and that people used to have to find a phone box when they were away from home to talk to the family.
In a word: history.
Dial-up modems are obsolete today. I don't think any company in the USA offers that type of internet connection. In my opinion, it is no longer useful for anything.
It is strictly inefficient for internet connectivity, but a modem can be configured, with the right software, to do fax functions: so you can send or receive faxes. In the days of those devices it was an auxiliary function which, in my case, was very useful. So yes, they can be used as a fax machine.
It can be used to make and receive phone calls with appropriate software. In certain workflows it may have some utility.