With regards to the hikmah behind the sending of the Messengers,
Shaykh Muhammad ibn Saalih al ‘Uthaymeen rahimahullaah
states:

So a messenger was sent to every nation from the time of Nooh up till the time of our Prophet Muhammad sall Allaahu‘alaihi wa
sallam.
 
And the hikmah behind the sending of the Messengers  was:
 
a) to establish the proof. He, the Most High, said:

Messengers as bearers of good tidings and warners in order that mankind
should have no plea against Allaah after the coming of the
Messengers…
Soorah an Nisaa (4) aayah 165

b) as a mercy – as per His statement, He the Most  High:

 And We did not send you except as a mercy to all
creation.
Soorah al Anbiyaa (21) aayah 207

 c) to make clear the path leading to Allaah, the Most High – because
the person does not know what Allaah has made obligatory in a detailed manner
except through the path of the Messengers.

 (Al qawl ul mufeed (1/27 to 28) of Shaykh Muhammad ibn Saalih al
‘Uthaymeen rahimahullaah, translated by Nasser ibn Najam)