They're usually sung at the beginning of the dance, when doing a 'walk round', kind of like an introduction, eg
'Oh where and oh where has my Highland laddie gone?
He's gone morris dancing with his tartan jock strap on.'
Sometimes it's used to link the dance with the tune, sometimes it's unrelated.
Here in NZ we start Vandals of Hammerwich with a verse from Flick the little fire engine! Don't ask me why. Anyone else do that or was it invented here?
Robyn