Variant covers for comic book issues have been a staple of the industry for some time now. The first comic book to have a variant cover was 1986’s The Man of Steel #1. While there were technically variants before this, they were all due to small differences such as distributor logo. Man of Steel #1 was the first to receive variant artwork, which is what variants are mostly known for nowadays.
Variants played something of a not unsubstantial role in the “spectator boom” of comics in the 1990’s. There are a lot of pieces to the spectator boom puzzle which I won’t go into here, but if you’re interested you can read this great article by Michael McCallum on ComicBooked.com.
To give a brief history of the boom, people suddenly got the idea that buying a comic and holding on to it for 10, 20, or 50 years would eventually put their kids through university or pay off their house. Big firms such as the Wall Street Journal and the New York Times wrote articles about the possible future value of these once unpalatable items and so people started buying comics by the armful.