Three situations justify it, and outside those three the money is better spent on speakers or the subwoofer.
First: a large or demanding room. Long listening distances (five metres and beyond), high ceilings, open-plan spaces that leak energy into the rest of the house. Sound pressure falls with distance, and reaching cinema levels at the sofa without strain needs headroom that a receiver's shared power supply may not have when all channels are working hard.
Second: hard-to-drive speakers. Low sensitivity, low or dipping impedance, or both. A speaker that dips to 3 ohms in the bass draws current the AVR's amplifier section was not really built to deliver, and the result is compression, hardness or, in bad cases, protection mode at the worst possible moment in the film. A dedicated power amp with a proper power supply is built for exactly that load.
Third: wanting real authority from the front three at reference volumes. Even in a normal room with normal speakers, some listeners want the front stage to sound effortless at high level, and moving those three channels to an external amp also lightens the receiver's load for everything else. Is it audible? At the levels involved, yes. At normal evening volumes, much less than the price suggests.
Where the rule stops mattering: living rooms of ordinary size, sensitive speakers, sensible volumes. There, a competent AVR does the job well and the external amp is a box you paid for and cannot hear.
The planning point is the one people miss. If you might add amplification later, choose an AVR with pre-outs now. Pre-outs are the line-level outputs that let the receiver act as a processor for whichever channels you hand off. With them, adding a power amp later is a clean upgrade with two cables. Without them, "adding an amp" means replacing the receiver, and that is a full spend for a partial improvement.
Check pre-out provision on the manufacturer's spec page for the model you are considering, or ask us and we will tell you which channels have them and whether your speakers are the kind that will ever need them.
Not sure what fits your room and budget? Share your room size and budget on WhatsApp. It is a two-minute answer with a floor plan.
