What's the speed callout by the PNF on airliners for?
This call serves two main purposes. The first is to verify that both airspeed indicators are showing the same speed. Since airspeed = life, it's best to find out that one of them is faulty on the ground rather than in the air. 

The second purpose is to distinguish a high-speed rejected takeoff from a low-speed one. Below 80kts (or 100kts or another speed set by the manufacturer and/or airline), a takeoff would be rejected for almost anything out of the ordinary. 

But rejecting a takeoff at high speed carries a lot of risk - like brake fires, tyres exploding, directional control difficulty, etc. So above 80 or so knots, a takeoff will only be rejected in the case of major failures. Some modern aircraft even inhibit minor warnings from appearing after 80kts until the aircraft is airborne, because rejecting the takeoff is not worth the risk, so don't saturate the pilots with unnecessary information. 

This page goes into some detail about what you would reject a takeoff for at low speed vs high speed. The criteria may vary slightly amongst airlines but the general themes are the same.