Qt widget (with layout) space. What is it? How to remove?

Question

I have written a program that uses widgets as container (for other widgets). Because the contents of the container varies along the program life, the container widget has a

Answer 1

The method I recall to control spacing is to add dedicated QSpacerItem items between the widgets.

But I'm first trying to figure out what w is doing. Why not call g1a_l->addLayout(t); directly?