In what situations are they useful?

https://github.com/ProseMirror/prosemirror-schema-list/blob/aa6f2a6c42ee0c23406b39a818d92f691068773e/test/test-commands.ts#L7-L15
Since tag returns pos, defining such a selFor() function has the advantage that the selection can be defined declaratively, increasing the readability of the test.

Ah, you are talking about tag, not flat. I guess adding that to the return type there could make this a little more ergonomic. Adding flat seems to serve little purpose though (and is in fact incorrect, since that's not a property being added to Node instances, but an internally used alternative return type)

@marijnh thank you for response, Certainly for my use case, only tag was sufficient and I could not think of a case where flat would be needed. removed it: a9cb60b