I guess what happens is the same as you advance Thrust a bit more - only this time it is done automatically so we will not forget to do it.
Why - due to the certification reasons. Engines must be capable to accelerate from 15% to 95% within 5 seconds in case of G/A.
And the airframe must be capable to achieve 3.2% landing climb gradient (all engines) within 8 seconds.
To achieve that we need slightly increased idle to allows engine to accelerate rapidly from idle to G/A thrust - since idle in all other situations is driven by fuel efficiency and is lower and acceleration would take more than 5 sec for engine and more than 8 seconds for airframe.
Although...even with approach idle active, if you are high and fast (read un-stabilised - but within the limits at 500'), idle may be below certification threshold. This is also parameter that shall be taken in to the consideration and that is why it shall be also monitored by FDM.
If the question was purely technical - what actually happens with FADEC or fuel control or thrust control or similar...than disregard what I wrote.