Well, "my" sources state they filled several roles including in fact those of hoplites in the flanks of a phalanx. However, one of the major rles they filled was the filling of gaps in the Phalanx. Whenever the Phalanx cohesion was threatned they came swords blazing and racing to strenghten the position. as they were heavilly armored and trained, they would have a lot of staying power so they could make a diference :)
In AS the Flanking role was atributed to the Agryaspidai Thorakitai as they were mean. I believe the EB Hypaspists make a hell of a historical version as you can have them fighting with spears (Hoplite wise) or using swords to prevent the Phalanx from crumbling.


Cheers...