It is generally not advisable to hold liquid STEEM for very long, because the price will usually be on a downward path due to inflation.

If you are interested in "investing" in STEEM for the long term, in hopes that the project will become a success, then powering up will give you "vests", which are protected from a large portion of the inflation by adding more STEEM to your vest over time.

Also if you power up, your votes will have a little more influence. Not much, but if you keep adding to it over time, it can start to add up.

It should be noted though, that any STEEM you power up is subject to the two year power down period if/when you eventually decide to cash it out.