Having a universal or "Standard" religion in a culture certainly enhances political control. The Roman Empire adapted Christianity, which started as a minor Jewish splinter group, because it saw its numbers of adherents increasing. As Rome began its long disintegration, so the adherents to the official "State Gods" fell off. Christianity provided a unifying cultural force that not only retarded Rome's fall, but maintained a culturally cohesive Europe until the Reformation.
The development of monotheism has arguably led to the rise of universal religions of salvation, such as Christianity and Islam. If I assume that there are many different, competing, and perhaps equally valid gods, I am perhaps less likely to assume that you need to worship the same god as I do. If, however, I assume that my god is the only one, real, true, legitimate, and worthy god, I am likely to assume that your god is false and that you need to be saved from such falsehood by the deity I worship.
In Biblical times, religion served many purposes, one of which was to differentiate tribes from each other; "My tribe is religion X, yours is religion Z, so we are different, while their tribe is also religion X, so they are the same as me." The need to be different caused the feeling that religion was something to feel superior about, and so the need to be "saved" came into play: "I can't be evil in my Crusades because I am Saving all these heathens from their wicked ways!" Additionally, homogeneous cultures stood a better chance of uniting against common foes, while diverse nations often fell apart because they could not get together with their mass differences.
There are only a few religions that do not actively practice missionary work with the need to "save" others.
I think sometimes that the religious aspect of the first wave of (Spanish) colonialism gets overlooked, due to all of the brutality and exploitation that took place in the first years of exploration and colonization. The crusader/missionary aspect of it was very strong, especially after the Protestant Reformation created a bit of a siege mentality for the Church. The reason I think it gets overlooked so often is that we ahistorically assume that people can't be cruel and exploitative and still have genuinely apostolic religious motives. If I had to assign a number, though, I'd guess more people were converted (to one degree or another) by trade and other avenues of cultural diffusion than coercion.
I would say imperialism had a profound effect on the spread of religions of salvation. The spread of Christianity, especially by the Spanish, but also by the French, Americans and British, was facilitated at a rapid pace from the 1500s through the 20th century. This explains the dominance of Roman Catholicism in Latin America, and the introduction of Christianity into pasts of Asia and Africa as well.
Early forms of trade and the spread of civilisations clearly are very important to the development and spread of the monotheistic religions, however at the same time we need to recognise that these religions also actually represent a much more sophisticated belief system with codes of values and laws that obviously separate them from primeval and animalistic forms of religion. This makes them intellectually and philosophically more robust than their counterparts.
I think that "globalization" in its early stages helped lead to this. These sorts of religions are different than the more localized pagan religions that dominated the world before the universal religions came to be. It is hard for a religion based on the worship of nature, for example, to spread from place to place since nature is different in different places. There also has to be an idea that all people are sort of similar so there's less of a competition between "my god" and "your god."
So, as barriers between people broke down (by trade, for example), it became possible for religions to arise that were universal.