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.
When that’s the case, the preprints would be just as short. We don’t really like unnecessary pain so we write short manuscripts from the beginning, if we plan to submit in such a journal. Usually, the longer versions get published somewhere else anyway.
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I don't know which field you're talking about, but in general, math and cs journals do not have 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