The TO/GA buttons on some aircraft actually have more functions that that. The 747-400 buttons act as a thrust limit selector as well, much as the thrust button does on the main control panel.
Pressing the TO/GAs once (if auto throttles are armed) on take off will select the pre set take off thrust - as selected in the pre-flight actions on the Flight Management Computer Control and Display Units. Pressing them a second time will give you Go Around power should you decide you need it.
On an approach, pressing them once will initiate a go around by giving you Flight Director commands and the power required for a climb rate of 2000' per minute. Pressing them a second time will give you full go around pitch up and power.
The Flight Director modes are automaticly swithched to TO/GA on pressing the buttons (having been automaticly armed on the approach), so they will maintain runway heading and the relevant pitch mode until others are selected - usually VNAV and LNAV if the procedure has been pre-programmed.
If an auto approach is flown, the auto pilot modes are switchcd to TO/GA by pressing the buttons, so the Go Around will be automatic.
The TO/GA buttons will only advance thrust by resheduling the Thrust Management System. To reduce thrust, you have to use the thrust switch on the Main Control Panel.