I have never used one, but I have talked to backpackers that cite smaller volume to pack and less weight. I am sticking with sleeping bags.

Last year I bought a new sleeping bag to replace one that is getting a bit tired after about 12 years of use. This is the one I bought.
https://www.rei.com/product/220973/s...-bag-mens-long

The above is the long I bought, the regular size has slightly less weight and packed volume:
https://www.rei.com/product/220972/s...eping-bag-mens

For comparison, that manufacturer makes a quilt to the same spec. Thus, you can compare apples to apples here. Quilt has slightly less weight, slightly smaller packed volume.
https://www.rei.com/product/220960/s...nitro-20-quilt

They list the quilt "fits to" length as between the long and regular sleeping bag lengths, that complicates a direct numerical comparison slightly.

I noticed the two different versions (bag and quilt) because I almost accidently ordered the wrong version.

I will let others that have actually used a quilt explain more since I have not used one. They probably have more reasons to cite.