Following the default taxonomies (categories and tags) and post types (posts and pages) that comes with WP, consider this scenario:
Imagine that I use posts to list IT contractors, and I use Categories for the Cities they are available to work in.
The idea would be that when editing the post and selecting the categories (cities), we could highlight the main category for this contractor.
This way, when browsing the list of posts (contractors) in the index.php or whatever custom page, we could give emphasis to the main city where this contractor is based on. Once looking at his full profile (say, the single.php template) then the other cities (categories) could be listed as well, or perhaps differently.
I can see workarounds with current WP features to obtain a similar functionality. For instance I could use custom fields in the post type for typing the main city. But what if Someone commits a typo, or writes the name of the city/location differently?
I mentioned posts and categories here, but I really wish this could be extended to all taxonomies (custom and default). So we can have the option to select a main term for a given post type (also, either custom or default).