Many organizations fall into the trap of trying to do too many things, especially at the same time. They try to be everything to everyone, and in the end, the organization waters down its message and leaves everyone involved unsatisfied. That is why there has been many pushes in the past decade or so to specialize and find a niche. In order to fulfill your mission though, you may need other goods and or services that you can provide, but do not play to your strengths. This is the perfect scenario in which to specialize.
Partnerships allow your social venture to specialize in what they are good at doing. By "outsourcing" other tasks, your organization is able to leverage the talents, skills, and resources of another organization in a way that furthers your mission more than if you had tried to accomplish the task internally. On the flip side, the exact same thing should be said about the partner organization and the value they receive from your organization. It can not and should not be a one way street.
The value of partnerships comes from the synergy that develops from good working relationships. Developing strong, strategic partnerships is a lot like team building, and as long as the two organizations trust one another, they have a strong base from which to operate, execute, and achieve together. The end result is always more than what either organization could have accomplished independently, and is exactly why you should specialize and develop partnerships.