To add to what HeartyMeatballs says, that speed call normally accomplishes three things:
1. The aforementioned boundary between low speed and high speed rejected,
Personally, I've never been a fan of this being one of the reasons. I say that because it creates "two V1 speeds" if the SOP is written a certain way. I'm of the opinion you should reject for anything you don't like when you're below V1.
2. The aforementioned pilot incapacitation check ("are you still with me"), and
A lot of SOP calls are designed to double as incapacitation checks. I try and teach in the sim that calls like "80 knots," "Heading," "Altitude," and the like should be taken by the PF to be a check that they're still in the land of the living rather than a commentary on their present flying ability (as some pilots take it to be).
3. An airspeed indicator cross-check.
In some aircraft, the airspeed indicator bottoms out at 40 knots, so a slightly higher airspeed allows for a decent reading prior to the cross-check.
Also. It is not always 80 knots. The company will pick the speed for the type being flown. I've seen 60, 80, and 100 knots in my travels, but they all accomplish the same thing.