According to the Quran and/or Islamic traditions, why did God create mankind?

Praise be to Allah 

Allah created the
jinn (type of spiritual creatures) and mankind to worship Him alone, to obey
His commands and heed His prohibitions. Allah says [in the Quran]
(interpretation of the meaning):

“And I (Allah)
created not the jinn and mankind except that they should worship Me
(Alone).” [51:56]

i.e., to believe
in His Oneness and to devote their worship to Him alone, to the exclusion of
all others. One of the conditions of worship being correct is belief in His
Messenger [Muhammad] (peace and blessings of Allah be upon him) and
following his example, for he is the example to be followed, as Allah says
(interpretation of the meaning):

“Indeed in the
Messenger of Allah (Muhammad) you have a good example to follow.” [33:21]

.