As a writer, I think the valuable lesson here is to read the script first.
Once you've seen the movie, the read is tainted by what you've seen.
Reading a script first allows you to envision the film in your head in your own way. It allows you to see what the writer is able to communicate to the reader - without the help of a director or actors or music.
Once you've seen the movie, the dialogue on page 27 will resonate with the inflections of Kevin Spacey or whoever played the part.
But when an agent or producer reads the script for the first time, he doesn't have that experience. The script must allow him to direct the movie in his own head. And cast it himself. And interpret the dialogue on his own.
And most scripts do not evoke a movie to the reader.
If
you read the script first, it has to do that for
you too. If it fails to evoke a movie, the script has failed. As most scripts do.
So, the experience of reading a script first will help you learn what works and what doesn't work in evoking a movie in the mind of the reader.
The relationship between scripts and buyers & sellers in the business is screenplay first, movie second. It's the natural order of things.
If you're a fanboy, read the script
after you've seen the movie. If you're a writer, read it first.