You bring up a good question. The truth is that if you are just interested in Steem as an investment then you can keep it in Steem, and you can retrieve it whenever you want. If you put your Steem into steem power you are locking it up. In order to get it you have to power down and the money will be paid back to you during 104 weeks (2 years). Let's say you have 1400 SP now then you will be getting 14 Steem each week for 2 years.
However locking it up gives you more voting power. This is very important on the platform because a lot of people will want to follow you if your upvotes are of higher value. Also you can upvote yourself so you are increasing your own value faster then more steem power you have.
In the end I think it comes down to your interest. If you are not planning to spend a lot of time on steemit and use your daily upvote power then it's better to put it in Steem.
Please let me know if there is anything which is not too clear .
Thank you my friend.
This was really helpful. :)
You are most welcome ! Glad If I could be of help :)