There are two reasons to power up.

• Curation rewards
• Inflation

Inflation is like getting interest on your powered up STEEM.
The "savings" account doesn't seem to work anymore,
so if you want to get free STEEM just for having STEEM, you need to have it powered up.

Curation rewards. Get STEEM for voting.
It was at the point of 1 MVest that curating became easy

Lastly, the rewards a person gets on their posts is made up of dozens of half pennies. So, every little bit helps.