People need to be connected. Evolutionary psychology teaches us that we are born dependent on other human beings, and remain so throughout our lifespans. The alliances we are thrust into (family) and those that we subsequently develop (friends, associates and colleagues) are vital to our success and well-being.
While it's common to speak of one's network in terms of size or power, alliances are a select ensemble of individuals within one's network that are unified and galvanized by a common interest, worthy cause, or singular purpose. They are predicated on achieving some immediate result, or attaining a larger goal. Alliances can be comprised of both the strong ties and weak ties that we hold.
Alliances, often referred to in business as "strategic" alliances, are filtered and formed via one's network. They are essential to the realization of professional accomplishments; proving that no man/woman is an island. It also proves that in all networks, crucial relationships crystallize based upon need; thus creating working/bonding opportunities for a vital few.
But allies, consisting primarily of intimate relationships between individuals, help to nurture and sustain us personally. Having vast alliances of powerful peers, or allies who empower you, can be just as effective. Seeking an ally is a prudent career move. Forming alliances is a smart business move. Having both makes you very fortunate.