What is the Benefit of Monotheism (Oneness of Allah) to the Muslim?
Right guidance in this world and salvation from eternal punishment in the Hereafter.
Evidence from the Noble Qur'an
{It is those who believe (in oneness of Allah) and confuse not their belief with Thulm (wrong) by worshipping others besides Allah) for them (only) there is security and they are guided}, {Soorah al-An'aam, verse 82]
Evidence from the authentic Sunnah
((The right of worshippers upon Allaah is that He will not punish those who worship none beside Him)), [Transmitted by al-Bukhaaree and Muslim]