Column sort order in CREATE INDEX statement? When creating an index one can determine if an index column is sorted using ascending or descending order. BOL says that

[ ASC | DESC ]
Determines the ascending or descending sort direction for the particular index column. The default is ASC.


What is the primary reason for choosing between ASC or DESC?
Since indexes can be scanned in both directions most of the time there isn't much reason for choosing DESC.  Some TOP queries could be helped by the DESC on some of the columns of a covering index but only testing will show if this helps.

One other difference is that the forward order scan can be run in parallel but the reverse order scan is always single threaded.