When designing a complex business process it may become too large to manage as one monolithic process definition. A sub-flow activity allows another process definition to enacted as part of another process.
It may be a prudent decision to design process definitions as a set of sub-flows regardless of whether there are concerns over size. This would allow sections of the business process to change without affecting others. Also the subflow processes could act as reusable components that customers can reuse in building their own higher order process definitions.