Subprojects for complex projects
I use the Subproject functionality infrequently. Where I have found it useful is for complex projects, where you can be moving simultaneously on multiple fronts at the same time. I used this, for example, when I was recruiting someone for a particular position and had multiple candidates. For each, I had a checklist of Next Actions that I was using, like read resume, call/email references, schedule on-site interview, book hotel, send out job talk announcement, solicit interviews, etc. I could do these for multiple candidates in parallel and it was helpful to have each candidate as a subproject.
phlow's example of project management above is a very good example where there are different elements of a large project that have many elements that can be moving simultaneously, though each with different completion dates.
Cheers, David.