On Reddit people are paying actual money to reward people with valueless internet points. I'm thinking if people would do that, they might as well power up and give rewards with value.

I think for a lot of people the reason could be to influence the discussion and press their opinion. It does feel good in a way when your opinion is ranked highest or a really stupid opinion is burrowed. Gives a sense of peace. Also a lot of people feel powerful when someone is grateful to them.

On Steemit the second element is a bit lacking as there is not a lot of feedback on who voted for you and what your vote did for another. There could be a dashboard with a podium for the people that interacted most with you for example.