Speaking for myself, I read a lot of comic books when I was younger, and it's a nice experience to see these characters that I grew up with up on the movie screen. Especially if they're done well. Because when I was growing up, there was an unwillingness and an inability to portray them convincingly onscreen - either they didn't believe they would be profitable enough to invest the money into them, or the special effects weren't good enough yet.
Now that it's been established that they can be profitable in a big way, the floodgates have been opened and they're making a ton of them. And it's great for the movie industry, because they now have a vast and proven resource that they can pull stories and characters from.
Superheroes are the modern world's mythology. They are also a type of fantasy writing similar to, but separate from the sword, sorcery, and dragons genre.
I think it's a mistake to dismiss all comic books as juvenile, however. Comic books are an art form just like anything else. There are good ones, bad ones, ones directed at children, and ones directed at a more mature audience. The Marvel heroes resonate with a lot of people because they were the first one to show superheroes with real world problems - for example, Spider-Man having girl trouble, needing money, dealing with his aunt's health problems, and trying to fit into a social hierarchy. Tony Stark/Iron Man dealt with alcoholism. Captain America appeals to our respect for the WWII generation and yearning for a simpler time, contrasted with the more complicated politics of today where there the heroes and villains are less clear cut.
A lot of non-superhero projects also started as comic books, like The Walking Dead, Sin City, 300, Men in Black, Preacher, Blade, Wyonna Earp, etc.