So Steemit Network starts from 0 supply and based on what is written on the whitepaper: there are approximately 800 new steems created every minute. Steem Power is the value of a unit that can be converted from Steem. For you to convert back from Steem Power to Steem, it will take 2 years, gradually install as much as 104x payments. Why is the 104x payment due to steem power that you convert to steem will be paid every week .. Because 1 year there are 52 weeks then 2 years is equal to 104 weeks. This process is called "Power Down".
The advantage of having Steem Power is that we will get Steem every time Steem New is created (still remember the point above, that supply steem will continue to increase every minute?) by converting Steem to Steem Power. We are guaranteed to get more quantity of Steem (in the form of Steem Power) as time goes on than you save Steem.