Why are some films grouped together in a trilogy listing?
I have grouped several films together in one single listing as a trilogy (or more in some cases), to reflect the likelihood of how people are going to view these films, and how they are marketed.
Many films, such as The Godfather, The Lord of the Rings, and Jean de Florette, make little or no sense watched on their own, and so the viewer has to watch all of them to get the full story. In other cases, films such Harry Potter and Star Wars lend themselves heavily to getting children (or adults) 'hooked' on a certain formula, coming back time after time, sometimes even developing obsessions. One of the main reasons behind listing films in this way is because many films are good at attracting viewers with a first impressive production and a degree of originality, only to provide sub-standard films thereafter.
Conversely, there are other groups of films that lend themselves to one-off viewing. These films are listed individually. They are films that usually take themselves a little less seriously and include the likes of Back to the Future and Indiana Jones.
Other films lie somewhere in-between. An example of this is the Three Colours trilogy of films. Watching these films together isn't essential, but knowledge of the first two may add something extra in watching the third. I decided to list these together because I think a minority of people will watch only one film.