Unfolding just the front handlebars gives you a 'Brommie on a stick' which is easier to drag along.
Gripping the bars also gives you some left/right leverage which means the bike is less likely to topple.
However you do it, the little rolling wheels are only any good for short distances on firm, level ground.
Not a problem, because a Brommie is so easy to unfold it's no hardship to do that and ride it.