Self-voting is not profitable at all if Steem Power is small, unless you have a large Steem Power, either through curating, posting in a certain period of time, or investing in Steem Power.
People like to do self-vote perhaps because of the nature of Steemit which really emphasizes social media that provides rewards, which is why small accounts still do self-vote even though the results are not too significant.
Ideally, it's not recommended to self-upvote your post. Community members don't like that activity wherein a person is upvoting his or her own post. Steemit is a platform wherein people like others post and reward them in terms of an upvote. I will not go into steam power and its influence. But when an individual starts upvoting themselves then we are just beating the purpose of the platform by upvoting ourselves.
Now when the Steem Power of an individual is less and if he is still upvoting himself it won't have any value - just that wherein the stats of the user upvotes is kept - "steemdb.com" it just increments to 1. Logically it isn't having any value when a person with very low Steem power is upvoting his/her posts and is just depleting their Voting Power.
Whereas if they read other posts and network, comment and upvote others' post it would definitely help them to grow and no doubt their comment could also get an upvote leading to rewards and higher SP.
It don't... unless!
They are sniping a curator they expect an up-vote from.
That way they get 75% + a little extra curation reward.