I just ordered my serger yesterday so I haven't used one yet but I did download the manual for it and have learned that you change the speeds of the front and rear speed dogs depending on the type of fabric you are working with.

This is what is in the manual for that for the Brother 1034D serger.

This serger is equipped with two sets of feed dogs
under the presser foot to move the fabric through the
machine. The differential feed controls the movement
of both the front and the rear feed dogs. When
set at 1, the feed dogs are moving at the same speed
(ratio of 1). When the differential feed ratio is set at
less than 1, the front feed dogs move slower than the
rear feed dogs, stretching the fabric as it is sewn.
This is effective on lightweight fabric that may pucker.
When the differential feed ratio is set at greater than
1, the front feed dogs move faster than the back feed
dogs, gathering the fabric as it is sewn. This function
assists in removing the rippling when serging stretch
fabrics.