With negation and exception, the item after illā explicitly identifies what is retained. In the explained innamā pattern, the final complement supplies the corresponding focus.
ConstructionFormal cue to restriction
مَا ضَرَبَ زَيْدٌ إِلَّا عَمْرًاʿAmr follows illā
إِنَّمَا ضَرَبَ زَيْدٌ عَمْرًاʿAmr is the final complement
Both can restrict the object to ʿAmr, but their grammatical configurations differ. Equivalent English translations do not erase that formal distinction.