Plane Handling are primarilary a Ramp / Cargo Company offering only Ramp / Pushback / trucking services, whereas Aviance offer the complete package of all of the above plus Check in / Baggage Tracing / Operations & Dispatch, it means that when tendering for contracts (ie) Qantas there is a bigger array of services on offer in the package and airlines don't have to split handlers or be self handling above wing, which is the case with all PH's current contracts at LHR (ie) Virgin, Saudi & Cathay, Plane previously handled Thai which was SGS above wing but when they began Ramp Operations lost it as a package to them.
The inability to offer passenger handling is probably a handicap as in addition to Aviance all other handlers, Alitalia (or whatever they are called now), SGS, Cobalt, Servisair, Air Canada, United, American etc all have that to offer.