I think books can be made into movies because the books themselves are already very theatrical- there's character description and development, descriptions of light/setting, and an engaging plot. And let's be honest, it's easier to use the ready-made plot of a book versus coming up with something completely unique.
I agree with what's been said about some book having fan bases and movie producers thinking this popularity will transfer over to movies, and for the most part it has. Nicholas Sparks and LOTR have already been mentioned, and we can't forget Harry Potter, Twilight, and The Hunger Games-- books and movies that have appealed to a wide range of ages.
To get to the heart of your question, I think that sometimes people will read a book simply because it's going to become a movie. Or they'll realize a movie is based on a book and THAT will cause them to read the book. In any case, the books are getting read, so I would say that movies are popularizing books.
On the topic of ruining them, I think books and movies should be viewed as two separate elements. Often, a lover of the book will hate the movie because details have been changed. But as viewers of media, we should know that screenwriters, directors, and producers cannot create a word-for-word adaptation of the book. The movie is a whole separate entity. It is based off of the book, but it isn't the book. For this reason, I see movies of books as something else entirely. Not something bad, just something different.