Air assist pressure by job

For cutting, it is the single largest quality upgrade available on a diode laser, and it roughly doubles effective cutting capability by clearing the kerf between passes. For engraving it matters less but still keeps smoke off the lens, which protects the optic and keeps power consistent through a long job. Most people who add it wonder why they waited.

Yes, in two ways. On acrylic, strong air frosts the cut edge and destroys the polished finish that makes laser cut acrylic attractive. On very light material such as paper, veneer or fabric, a strong jet lifts the workpiece off the bed, which ruins focus and can start a fire when the sheet flaps into the beam.

It reduces flare ups considerably, because it blows out the small flames that form at the cut line and stops them establishing. It does not make a laser safe to leave alone. The fire risk that matters is the one that starts after a flare up you did not see, which is why staying in the room and keeping an extinguisher within reach is not optional.

A small compressor with a tank gives steadier pressure and is much quieter in use, since it only runs to refill rather than continuously. The downside is cost, space and the need to drain moisture from the tank, because water carried into the air line lands on the workpiece and the lens. An aquarium style pump is the usual starting point for good reason.