I use the smooth band just below the cap thread to engage the silca pump head gasket so it 1 saves wear on the pump gasket
and 2 makes it easy to pop the pump head off when Im done ..

Sewup tires the stem is glued to the rim with the tire so pushing the pump head onto the stem is resisted by the glue..
the ring nut serves that function on a clincher tire when tire completely deflated ..

some obcessing posters, here, seem hate the ring nuts as 'too much weight' ,, they may be happier with out the ring nut threaded stems .