Overstuff has a few handy uses.

The first use is if you wish to increase the temperature rating by only a few degrees such as a 0*C quilt to -3*C instead of opting for the -5*C.

In this case adding 50g of overfill would achieve this.

Generally every 20g of overfill will increase the comfort rating by 1*C

Another use of overfill is to reduce down shifting. I design my quilts to already be very overstuffed so down shifting should not occur. However, some people may have previously had bad experiences with down shifting or may be prone to excessive down shifting in which case extra overfill will help reduce this.

The reason that overfill helps reduce down shifting is that for every 5*C the quilt temperature is increased (like 0*C to -5*C), the minimum baffle height (the mesh baffle height) is increased. But if you order overfill, this minimum baffle height is not increased but there is more down, leading to a more stuffed baffle.

For information on Fill Power