I think you're misunderstanding. Consider a merge like the one below. Each line is a lane, and the direction of travel is upwards. This is sometimes how an on-ramp ends, for instance.

     ||
     ||
     ||\A
    C|| \B
     ||  \

Cars A and B are in the \ lane, and Car C is in the leftmost | lane. The \ lane has a yield sign.

The driver in Car A is a new driver, and is uncomfortable merging. The lane they would be entering is clear, but they're spooked by the car in the far lane and stop unnecessarily at the merge point.

The driver in Car B behind them is following closely and is preparing to merge by doing a shoulder check. They are not looking forward when the car in front of them stops. If the way had been blocked they would have been expecting the car ahead to stop, but they had seen it was clear.

The driver of Car B is at-fault in that collision. They were depending on the driver ahead of them to behave predictably, and they were not driving defensively enough. A warning noise would probably have triggered them to slam on the breaks and might have prevented the collision. Automatic emergency breaking would do the same, perhaps more reliably.

(This was a real accident. I was a passenger in the lead car.)

I was always taught (in the UK) to check your mirror before signaling, and then make a final check before the maneuver.