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To read the content before posting
as a lot upvote without reading..
When voting on Steemit you get curation rewards for upvoting content early.
However, there's a rule in place reducing your reward if you vote too soon. This uses a linear model where you get 0% reward if you vote immediately as its posted, 50% if you vote after 15 minutes and 100% if you vote after 30 minutes or later.

Okay, I guess that makes sense as far as incentivizing people to actually read the content, the balance needs to be struck somewhere so 30 mins seems as good as any.