As discussed here, AR15s were primarily designed to take less time to unlock, which allows for the carrier to start moving later in the gun’s cycle, which is extremely advantageous for accuracy and reliability.
As part of this overall strategy, there is no “runway” built into the carrier’s cam slot, meaning the nanosecond it starts moving to the rear it begins engaging the cam pin, thus unlocking the bolt.
The issue that arises is that carriers bounce off the breach when they slam home, and that few millimeters of bounce, as stated, will start to unlock the bolt.
Note that ALL automatic guns suffer from bolt bounce. Even the legendary AK47, given directly to Mikhail Kalashnikov by the Scythian gods themselves, suffers from bolt bounce. Yes, you heard that right, even Mikhail Kalashnikov can’t defy the laws of thermodynamics.
That few millimeters of bounce is compensated by a few millimeters of “runway” in the carrier’s cam path, meaning the cam path doesn’t actually engage the cam in the bolt within that distance.
The tradeoff, though, is that the carrier has to start moving even earlier to compensate for that margin, meaning you have to tap the gas from the barrel earlier, the piston moves earlier, etc. All of which is bad for reliability, durability and accuracy.
The AR solves this problem by solving bolt bounce altogether. The buffer is essentially a hollow metal tube with a weight that slides freely back and forth inside of it. It’s literally just a dead blow hammer.
Besides the potential for an out of battery detonation, bolt bounce is also not conducive to reliable cycling, especially in multi-shot modes. If the hammer outruns the carrier’s return post bounce, it will result in a stoppage. Fast hammers are preferable for accuracy; the longer the hammer takes to detonate the primer, the more time there is for the shooter’s muscle movements to pull the gun off target.
Long story short, the buffer isn’t a bandaid for an AR specific deficiency, as is often claimed. Annoying as it is, the buffer, bane of foldy boi enjoyers everywhere, is part of why the AR15 is superior to everything else.