As-Salaam alaikum,
Have you noted it that whenever the Prophet, Sallallahu alaihi Wasallam, focused on his human Zahiri (outward) manifestation, he used to say : 'I am a human being like you'?... but when he focused on his batini haqiqat (inward realities), he used to say : 'I am not like any one of you, I dwell with my Lord at the hearts of veiled nights and He nourishes me.' (Bukhari and Muslim).

In Surat Fathi of the Holy Qur'an, it is says: O My Noble Messenger, "Indeed We have sent you as a Witness, and a bringer of glad tidings and a Warner" (48:8)

In the Qur'an, Allah Ta'ala confirms that on the Day of Judgement, for each community their respective Prophet will be their witnesses and Prophet Muhammad, Sallallahu alaihi Wasallam, will be witness for all of them. This is found in Surat Ahzab ayat 45 and Surat Muzammil ayat 15. These Noble verses/ayats tell us that all human beings who have appeared on earth from the beginning and those who are now present, those who will come until Day of Judgement, Prophet Muhammad, Sallallahu alaihi Wasallam, will be a witness to all on their faith and actions.

But for a person to be a witness, he needs to be present and observe the actions both outward actions and inward state of heart (for faith is an inward function)

So, this proves that the Prophet is bestowed the quality of HAZIR and NAZIR (omnipresent and omni-visionary) by Allah, the Owner of all Power and Glory.
Suprised? Look at it this way... Allah as Creator doesn't work without medium.. eg He distributes the sustenance of creation through the parents or someone else. He provides visible light on earth through the sun, similarly His showering of Mercy upon the entire creation is through a medium He chose as Sayyadina Muhammad, Rasulullahi, Sallallahu alaihi Wasallam. Thus the Prophet is the sun of Mercy, he is the distributor of Rahma.