The vast majority of active users on STEEM are plankton, with STEEM Power ranging between 15-498-ish SP. I don't know the numbers, but my guess, there are for more plankton accounts under 250 SP than there are those above it.

So, as hos been previously mentioned in the other answer, the upvotes of these accounts are very small, down to $0..001 in some cases. They can't really make much in curation rewards, especially on a comment that has no other upvotes, so they spend most of their time upvoting posts that already have someone upvoting it above the dust threshold.

A lot of these accounts, therefore, end up in some kind of curation trail, or autovoting, to try to maximize as much as they can, their curation earning potential.

For my part, however, I've never truly understood why users with so little SP would want to spread out their upvotes by reducing their vote weight to percentages of 10 or below, rather than upvoting less, but doing it at 100%. I imagine the shotgun approach is because it's difficult, based on the way curation works, to hit posts that will actually earn enough for any curation to trickle back to the small SP account.

I have to admit, it really is rather disheartening to see so many upvotes on a post and see so little in rewards waiting next to it. Even larger accounts are trying to spread their votes around. I'm glad they're trying to help so many people, but ultimately, growth is stagnating as all accounts are trying to raise everyone else.

Whether it will work or not, is still to be seen.