When using custom sort, I expect created or moved notes to consistently appear in predictable locations, like other sort types.
For creating notes with custom sort, placing notes above or below the current note empowers creating notes where desired, minimizing reordering. This also keeps the user's place in the notebook, which informs reordering. If my mental model is correct, incrementing or decrementing the selected note's order value and assigning it to a created note should achieve this effect.
For moving notes with custom sort, placing notes atop the list aligns with how switching notebooks selects the first note in the list.
Like many notes per notebook, limited screen real estate is another use case. Examples include using the mobile app in landscape mode, the Note List Preview plugin, a stacked sidebar and note list, etc.