The very first mass use of bitcoin was probably on Silk Road, a drug dealing marketplace. People used BTC there because of its technology; pseudonymous nature of bitcoin. It has created a lot of attention which later have turned the price higher and higher; got more attention.
Now BTC is popular for both. We have a lot of places to spend BTC. It's technology is developing consistently.
Please know that Silk Road =/= drug marketplace. I don't know, but I tend to feel that it is a bit unfair equating Silk Road with illegal drugs. Silk Road = free marketplace. And the
free here is the
free in its true sense. It is not the
free that we know in our society nowadays where restrictions are found everywhere. The tendency with a free market is that there are goods and services sold and bought that are considered illegal outside. And that's probably the reason why it was operating as a black market in the deep web using TOR.
That market was perhaps the earliest and biggest market where Bitcoin is the primary currency. And Bitcoin thrived there due to its technological design. So, I guess it is technology that really comes first before people are attracted to it. But of course Bitcoin's technological features were already discovered and known even before Silk Road came into being.