Probably it's because it's part of the Airbus Golden Rules. "Understand the FMA at all times".
That golden rule is fine, and actually the same on other types as well, for example boeings as long as you use the FD or AP. The need to call it out is simply an ass-covering exercise though. Although there is some bonus to it, it assures a regular wake up call when fatigued, except if your colleague decided to go raw data which does away with the FMA callouts.