To clarify a bit for those that are new to STEEM I will add this detail.
When STEEM pays out 1 $ worth of rewards (whether from posting, commenting or upvoting) it simply creates the new units "out of thin air". This has the effect of diluting everyone's STEEM holdings since the supply has increased but your personal wallet's balance has not.
The way to protect yourself from this dilution is to Power Up.
What does this do you might be axkin '?
Well, for every 1 $ worth of STEEM created to pay for content & curation there are 9 $ worth of STEEM created to send to those that have powered up.

So you should only hold STEEM if you are planning on selling it, or sending it to someone. Do not hold it for a long time since it is being diluted little by little by everyday's payouts.
Power it up and you will lock in your value.

Hope this helps complement the main post!