Why do candidates need a majority of the delegates?

[LiberteToujours]

I don't really understand the concept of a brokered convention. Why isn't the primary process treated like a normal election where the person with the most number of delegates wins? I can imagine one explanation being so that candidates A could give his delegates to candidate B, allowing him to beat candidate C, but I don't really know for sure. Is a brokered convention still useful nowadays given the primary process, or is it a relic of the past that we haven't dared change?