You can sell Steem for btc and then usd anytime, but Steem Power is locked up for 13 weeks, if you want to convert Steem power to steem you can do this in roughly even payments over 13 weeks, called powering down from your wallet. Also if your account has not received approx 30 SP, you will get a message like the below
Unable to power down (ERROR: Account requires 10x the account creation fee in Steem Power (approximately 30 SP) before it can power down.)
So your 1 Steem investment is worth
0.55 Steem on post pay out
- 0.042 steem per week for 13 weeks
Have you made a profit? It's complicated.
I guess the idea of promoting good valuable creative content is that other people will see it and follow you and if your content is interesting and worthwhile they may follow and update your posts in future for free. Also if you offer something to the world, a business or community idea you can sell this aspect of your company to people who follow your blog.
Also as you increase your reputation number you are signalling that you either are a good content creator or have been willing to invest in steemit to pay for promotion, this can help others to choose what content to watch and read, sort of a snowball effect.
tldr: no such thing as a free lunch but yes promotion can be worthwhile.