If you need a certain density.. usually lower than stock pieces come in, then you need to order custom. Stock density is really high for me, and while it can be thinned out, it's still not the same.

Also, custom means that it will fit your head better/less chance of wrinkles.

Also, hairlines for stock pieces are always a certain curve, and if you need something different, then you need custom.

Custom can come in every type of base, stock usually only comes in french lace base.

Dis-advantage of custom is the wait time. It's always good to have a new stock piece as a backup.