With each episode typically ending in a cliffhanger, they encouraged regular attendance at the cinema, and the short running length kept down the cost of each installment, and the number of reels needed to show them.
When the initial film has been highly successful, additional installments may be produced concurrently, taking advantage of economies of scale and the availability of actors and directors to facilitate production.
The Matrix, Back to the Future) A common reason for splitting a film has been to accommodate an extended running time; many people would find it uncomfortable to sit for a single three- or four-hour presentation.
Kill Bill) In the 21st century, it became increasingly common for big-budget films – usually those based on novels which might otherwise have to be substantially condensed, but especially the last in a series – to be released as multiple features.
[1] In 2012, the Australian Broadcasting Corporation called it "a recent Hollywood trend of splitting a single book into multiple movies to maximise box office returns from blockbuster franchises".