There are many film and movie databases probably with the IMDb being the widest known one. Many of them, the IMDb included, license their content under a proprietary license. This means that you are highly restrictive to use their content/data. You might not be allowed to share some of their data about your favourite movie on your personal website, for example. Or you must use the provided API, where you are dictated the terms, whether you like them or not. And if there are changes made, you simply have to accept and go on with it.
This is where the OFDb is different. As long as you attribute the source, you can use the provided data without restriction. You can even copy, modify and redistribute the data. This gives you essential freedom that is not granted by proprietary licenses.
But why the Open Database License specifically, and not some Creative Commons license like Wikipedia, for example?
Every license serves a specific purpose. The GNU GPL was created for software and the Creative Commons is to share your “creative work.” The ODbL on the other hand, is used for a database, and not for the content, per se. And this is exactly the reason why this license was chosen.
The OFDb is seen as a database (a collection of data). It contains complex relationships between its content like companies, actors, release dates, and so on. The focus is less on “in which movie did actress X play” (even though this might be the main point for many or even most users). Rather it is considered a collection of movie and TV show related data, that can be used in any possible way—whether it’s in a mobile app, a website where users can rate movies, or virtually any project where you might want to access the OFDb’s data.
With that in mind, what you see on ofdb.cc is rather a front-end to the OFDb’s data, and not the OFDb itself, so to speak. Of course the website is quite important since it is the also the place where you add and maintain all data, but it is also possible to build alternate editors to do the same (due to the flexibility of Drupal’s API that is in place).
So in short: An open license was chosen to give control to the user, and the ODbL is a fitting license since the OFDb is seen as a collection of data (database) and not simply a website that lists movies and TV shows.