Simply put --, when you need to navigate between more than 2 directories, usually several times back & forth, as cd - just won't cut it with anything beyond 2 folders.
So, for example, instead of trying to re-come up with previous long paths by looking at your buffer's history or tab-completing a long pathway you simply stack the important ones up and if needed you conveniently move to them by their number alone. Rotating between complex directories structures and long paths becomes slick and swift.
The builtins also allowsallow you to re-order the stack or pop out the directories you don't need anymore allowing flexibility in your work flowworkflow.
Directories stacking can also be used in scripts similarly for operations that span several directories.