Occasionally people may self vote their own comment because they want to raise visibility eg if I call out an abuser on his post I self vote to make people aware.
I think some newbies do it coz they think it looks good to have more upvotes aka likes on FB/IG in the web 2.0 world, and they don't realise doing that on Hive has undesirable effects. But mostly it's people being greedy wanting more rewards.