Using delegates in iOS and Cocoa is a very basic and fundamental part and we use them quite frequently in our codes. As like in business, cocoa uses delegates as a formal way to pass data(job) from one object(client) to another(vendor). In business, you want to do make something but you need raw materials to do so. So you ask the supplier to give you raw materials and you sign a contract for that. Same thing goes in cocoa, a class that wants to perform a task that depends on the response of another class acts as the delegate to the later. The first class wants to be the vendor of the later one by signing a contract called protocols (as defined in cocoa).
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