For collectors, the first appearance of a character or team is a highly sought after prize. The comics that contain the first appearance of a classic character are often worth more because of said appearances. The first appearance of Superman in Action Comics #1 is said to be worth more than half a million dollars.
It can also be used to mark the change of a character, such as the case of Green Lantern. There is the first appearance of the Green Lantern character in All-American Comics #16, or it could refer to the first appearance of Hal Jordan as the Green Lantern in Showcase #22, or the first appearance of the Green Lantern Corps, also in Showcase #22. As one can see, the term can be applied in many different areas.
The term eventually became part of the marketing lingo for comic books. Covers were plasted with the term "first appearance" and others like it such as "first issue," "debut," and "origin." Many modern day comics use these terms to generate hype about their comics and events that are contained within. This doesn't necessarily make the comic worth more, as only demand, hype, and availability can truly determine the market worth.