The purpose of last verse harmonisation is to add interest, variation and excitement to a hymn tune.
Usually the organ accompaniment to the last verse of a hymn tune will be heavier (in musical terms) than the standard harmony.
A notable example is David Willcocks' harmonisation of the seventh verse of the famous hymn tune "Adeste Fideles".
Organists will usually vary their use of stops throughout a hymn tune, including changing them for the last verse.
Sometimes hymn books and other compilations of tunes (such as Carols for Choirs) that publish last verse harmonisations will indicate suitable stops to be used, but this is ultimately a choice for the organist.