It is also valuable for scientists as it is often a 'directors cut' version of the paper. Journal submissions are heavy edited and shortened to fit into the page limits.
By the way, one of my favorite pastimes is to download the latex source for papers on arxiv and read all the commented-out stuff.
% we should make sure this theorem is actually true
A lot of physics journals do. Anything ending in "Letters" (e.g. Applied Physics Letters, Physical Review Letters".
Science has a word limit per article.
Nature doesn't have a hard limit, but if it exceeds 6-8 pages, it needs to be exceptional.