The Spiritual Purpose of Muslim conquests:

The effect which Islam exerted on the education of its followers in the first period of Islam by the purification of their souls, polishing of their minds, the sincerity of their beliefs and actions and sincere love and well wishing for the mankind ensured that their participation in the military campaigns against other lands was not primarily motivated by any worldly ambition. 

Rather, it was the compelling desire to propagate Islam, to enable it to take firm root in these lands and to organize and administer the newly-conquered territories. 

It was the wish to solve the various problems of these lands, and any new problems which might arise, in accordance with the true teachings of Islam. 

The Muslims were not motivated by any desire to dominate those peoples or to acquire their wealth, nor were they attempting to escape from the hardship of life in the desert, as has been maintained by the Orientalists.

Mission of Muslims is to liberate all of Humanity:

نحن قوم ابتعثنا الله لنخرج العباد من عبادة العباد إلى عبادة رب العباد، ومن ضيق الدنيا إلى سعة الآخرة، ومن جور الأديان إلى عدل الإِسلام

Al Tabari narrated that just before the start of the important battle of Qadisiyya, Rabi` ibn `Amir رضي الله ﺗﻌﺎﻟﯽٰ عنه  entered the court of Rustum, the Persian commander, who asked him: “What brought you here?”

Rabi` رضي الله ﺗﻌﺎﻟﯽٰ عنه  said: “God sent us, and He brought us here in order to lead whom He wills from the worship of man to the worship of God alone; from the narrowness and oppression of this world to the vastness and abundance of the hereafter; and from the injustice of other religions to the justice of Islam.”

What Rabi` ibn `Amir, رضي الله ﺗﻌﺎﻟﯽٰ عنه  the representative of the Muslims, said to the Persians was not only the expression of his personal feelings. Rather, it voiced the thought which was predominant among the Muslim leadership and most of the Mujahidin. 

This does not exclude the possibility that some of the Bedouin who had participated in the military expeditions had been attracted by the prospects of material gains and booty, in addition to the desire for jihad; but these Bedouin did not represent either the leadership of the movement or its motivating spirit and were only a small minority in the Muslim armies. 

Adapted from 'Madinan Society at the Time of the Prophet' by Dr Akram umari