The normal layout, from a typographical point of view, should be
\flushbottom. This makes the last line of each page be at exactly the same place on the sheet of paper, and it is the default in LaTeX.
It works fine for pure text, with regularly spaced lines (say literary text). However for non-literary books, with figures, tables, float elements, displayed math formulae, &c., it may come problematic. These elements introduce ‘rubber (vertical) lengths’ (i.e. adjustable lengths) to try solve the problem, and in some circumstances it may happen this mechanism leaves oversized white spaces in the page, especially when the author forces the placement of a float element at a specific place with the
So one of the possible solutions is to use the
\raggedbottom directive. All pages will not necessarily have exactly the same height, but ‘almost the same height’. The difference will be noticeable mainly if it happens in an even page and the following odd page. In most cases it will be imperceptible to the eye.