it counters the nose up effect of setting engine takeoff thrust
one point of note, if you look at the FCTL ECAM pg as you do the flt control checks, half stick down already gives full elevator deflection; hence full stick is not actually achieving anything.
changing from half to full stick down in the conditions on topic, I am told, is a procedure that all Airbus perform, and on the 330, there is a difference, but not on the 320. A 330 jock may be able to shed some light on this information.