Why should APO (finished goods) and SPP (Service Parts) be on separate boxes ?

Hello,

Could someone tell me why for a global company the APO System (for finished goods)

and the SPP System (for Service Parts) should reside on two different landscapes (SCM boxes) ???

SAP suggests that because it's Best Practice how Volvo, CAT, Ford are doing it,

but I didn't find a statement in a document.

Thanks

Jo.