I actually studied comic books back when I was in game design and I even took classes in a school co-owned by someone who, at the time, was the artist doing Justice League of America.
The strict and boring answer of how graphic novels came to be was that they were supposed to be limited series (as in with a start and an end, and not keeping going forever) , separated from the main comic continuity if they used an existing active character (I'm looking at you, Killing Joke, who was never supposed to be part of the Batman canon but was so popular it became). And, the biggest difference is that they are supposed to come in "premium" format, with special binding (hard cover, etc).
Usually they were also the ones allowed to explore more adult and controversial themes that are not in the comics, but with time this line was crossed by some mainstream publications.
After that, there is also the library and publishing aspect as
@Narisucks mentioned. It is good publicity/marketing to keep that.
Also keep in mind that in Europe, the graphic novels were slightly different. Some series like Corto Maltese and Asterix were published in graphic novel format as the adventures were basically , as the name would imply, "graphic books".
Oh yeah, to make it clear: I'm not disagreeing, I'm just giving a tl;dr of why it exists.