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).