Complete uppers are helpful for those that are looking to swap to a new caliber on their AR-15 or AR-10 quickly. If you currently own an AR-15 platform that shoots 5.56NATO, you can simply buy a .300 Blackout complete upper and attach it to your AR-15 complete lower. Same with an AR-10 that currently shoots a .308 WIN. Buy a new 6.5 Creedmoor complete upper and attach to your AR-10 lower.