why did u use update stategy in your application?

Update Strategy is used to drive the data to be Inert, Update and Delete depending upon some condition. You can do this on session level tooo but there you cannot define any condition.For eg: If you want to do update and insert in one mapping...you will create two
flows and will make one as insert and one as update depending upon some condition.Refer :
Update Strategy in Transformation Guide for more information
I have used in the case where i wanted to insert and update therecords in the same mapping