Each script in a Flash movie is a separate entity. In Macromedia Director and some other multimedia programs, a script is similar to a Flash symbol, and it can be used multiple times from a single instance. In Flash, though, to use the same script on multiple movie clip instances, you need to copy and paste the script onto each instance separately.
This is simple enough when only a few instances are involved. When you've created a standardized button script, however, and you need to use it on 40 or 50 buttons in your movie, it can get tiresome.