TeX/unskip

Synopsis
will remove the last glue from the current list.

Description
IF the last item on the current list is glue,  removes it; otherwise   has no effect. It does not remove kerns.

The command may not be used in vertical mode if the main vertical list-so-far has been entirely contributed to the current page [280]. However, \vskip-\lastskip has almost the same effect.

is used where you want to allow some flexibility in the input syntax.

For example, it's used in some of the centring code to avoid a space at the start of a line messing up the alignment.