Trivial example 1: every morning my list gets updated with a "morning routine" checklist that includes subitems for brushing teeth, taking a shower, eating breakfast, and packing lunch.
Trivial example 2: when adding an ad-hoc research todo, I want to use a checklist that includes subitems for both reading and for taking notes.
Trivial example 3: I want a recurring todo every other weekend to trim my dog's nails.
There are a bunch of other features that existing apps do reasonably well, with various tradeoffs (eg. features around urgency and/or expiration, or categorization, or ...), but I haven't found an app that does the core I described above (checklist apps don't do ad-hoc todos, for example).
All of which is great for making sure you do something the same way without skipping steps every time, but prevents you from completing both checklists and less structured todos in the same context.