The M-4/16 has a bayonet lug just below the barrel due to a requirement that it has the ability to mount a bayonet. Unfortunately, that lug gets in the way when you want to use an extended handguard.
When Geissele was developing their submission for the cancelled M-4A1+ RFI, they decided to develop this Bayonet Mount which attaches to a Picatinny rail section at the bottom of a handguard.
This frees up a lot of real estate.
