The idea really stems from the fact that the vendor that is going to do the best job is the vendor that has its corporate strategy, its capabilities, its resources, its assets, its soft skills, and its management preference in a much better alignment with your stated outcome than the other vendors. The idea is to assess these against your strategic outcome.
By the way, we talk about outcomes as if they're static. Your outcomes within themselves are not static, but maybe you have some target outcomes based on your current understanding of what you want done.