Spacers are not sizers per se, but are used in sizers for one of two purposes.
First, a spacer can establish a particular area of empty space on a form to keep various other visual elements from appearing cluttered by visually separating them.
Second, a spacer can take up an "empty" space in a grid sizer when appending a series of items.
Spacers are not physical objects; rather, they are rules that tell spacers to allocate a given amount of blank space into their layout.