Allah is the Rabb, and the Messenger ﷺ is His ‘Abd.
The preceding ayah uses رَبِّكُمْ, “your Rabb.” Rabb and ‘Abd stand in a direct relationship: Lord and Master, servant and slave.
Calling him رَسُول refers to revelation coming to him. Calling him Allah’s عَبْد presents him as the model of servitude to Allah. The phrase يُنَزِّلُ عَلَىٰ عَبْدِهِ already establishes his Messengership while adding the honour and model contained in ‘Abd.
Freedom from the pressures of society
Freedom from enslavement to personal desires
Freedom from every competing source of bondage
Allah’s pleasure becomes the highest consideration in every matter.
True servitude unchains the human being. Cultural and societal norms carry no authority when they oppose the attainment of Allah’s pleasure. Servitude to Allah is therefore a position of great honour, unlike every other kind of slavery.