There were some good reasons. I travelled for work to construction sites, and they had local dial in numbers in many places. I could check email from the construction trailer. They even had a 1-800 modem line (You were very time limited, 3 hours a month or something.)
I thought it worked pretty well.
Ubiquitous Broadband killed it.
Not much later, they changed their technology so that they worked through a conventional IP address, so you could fire up AOL, go online, then open a conventional browser or e-mail program. One of the e-mail programs had a facility for fetching your AOL e-mail, so it was like you were a regular Internet user.
Alternatives for reaching the Internet involved a lot of delving into settings files and setup scripts, which just didn't seem like a good use of my time. My friends who did things the "correct" way seemed to have a lot more trouble and maintenance effort. I listened to their adventures the same way I read Jerry Pournelle's "chaos manor" columns, feeling a mixture of amusement and pity.
Now I have to have an ISP who lets me open any port I want, host what I want (within reason for a home connection) and just stays out of the way.