you can use the exchange . (by using the Buy or Sell option which also pops up in your SBD drop down menu)
https://steemit.com/market
and sell your SBD at any time you want. (or rather sell SBD to BUY the Steem)
after this you could simply power up. (which means you convert your steem to vest)

[to answer your question]
if you convert your SBD to steem , you eliminate the impact of your orders on the markets.
so this is useful if you dont want to influence the price while selling alot of SBD at once.
the actual conversion rate is simply the average price of the upcoming 7 days.
Update:
after Hardfork on december 6th 2016 this conversion rate is changed to the average of 3.5 days.