sometimes they're quite good
I agree that novelizations seem redundant. But on some occassions I think they're actually better than the film. I remember they released the novelization of Last Crusade a few weeks prior to the film. They usually do as it's a form of publicity. Stupidly, I read it right away. Just couldn't help myself. I was actually disappointed in the film because of it. I've since learned to love the movie on it's own merits, but at the time, I can remember being quite underwhelmed.
There's another good example out there that is not Indy-specific, but another case where the book was much better than the movie. Remember The Golden Child with Eddie Murphy? Awful film, but an excellent book. Much more serious and exciting than the movie. Another example: remember Jewel of the Nile, the awful sequel to Romancing the Stone. It was a very exciting book.
Also, novelizations will often include original bits from the script that get cut or changed in the editing room. For example, the second grail test where Indy drops through the floor? Originally a large spider jumped up through the hole. There are even publicity stills that show that the scene was filmed for the movie, but at some point they changed it. It remains in the book though. As for The Golden Child, the author probably wrote the novelization based on the original script, which probably changed during filming as it quickly became an "eddie murphy movie." That's speculation of course, but I think it makes sense as the tones are so radically different.
All that said, I'm pretty selective re: which novelizations I bother with as most are not written by top flight authors. I think that's why I'm so pleased with their selection of James Rollins to do the new adaptation. He's quite good, so my hopes are high.
walker