Back in the day (and long before the Internet) if you didn't buy Trek merchandise in the store then the only other way was either by (snail) mail order or at a conventions dealer room.
My very first convention was Toronto Trek '76 at the Royal York Hotel when I was 17. I went on a Friday afternoon and it was a wild experience seeing so many Star Trek
fans gathered in one place---it was mind-boggling for someone who felt like he was pretty much the only one around. Even so I didn't get to attend another convention until about 1990. A few years later I attended Toronto Trek every summer for several years throughout the '90s. A lot of that time was spent manning a fan table where I and two friends peddled our fan writings. The rest of the time I really enjoyed sitting in on discussion panels.
I drifted away from the conventions when the feeling started to change. The convention was getting smaller and there was more and more interest on things that didn't interest me. That said one of the great appeals of Toronto Trek was that it was organized and run by fans---you felt welcomed. I tried attending one of the bigger media driven conventions, but I never went back because the feeling was totally different with seemingly everything carrying a price tag. Toronto Trek felt like what the first conventions might have been like: a gathering place for folks enthusiastic about the same thing and eager to share their enthusiasm with others. The media type conventions make me feel like it's just an oversized dealers room and not a cheap one at that. And candidly with what you can get online at any time a convention's dealers room comes across a rather redundant.