Lots of ballads end with the 2-3 stanzas about the rose and briar twining over the lovers' graves, Barbara Allen, Lord Lovel etc etc.

The example given by Michael above is actually part of a 3-4stanza run which can occur as solo stanzas or in almost any combination where the needy lover asks for a boy to run a message when one suddenly appears and runs,jumps, bends his breast and swims, leaps a castle wall/gate, gives the message to a lover, followed by the lover asking what's occurred and then the denial by the messenger.

There's multiple usage of sitting in bowers sewing a silken seam, and tirling of the pin. There are thousands of them in the Child ballads.
Lily-white hands and steeds. Looking over left shoulders,

When mass was sung and bells were rung and all were bound for bed, is a cliche.

A lot of it is there to help the singers recreate the ballad as they go along using familiar phrases. On the other hand it has also been used nauseously by antiquarians trying to lengthen their particular version.

BTW the more technical term is 'Commonplace'