1. Will I trust this person to do their job competently?
2. Would I trust this person not to defect in a prisoners dilemma situation?
The second is what you need the personal connection for. There are two ways to work on a project, to make sure it has the highest chance of success, or to make sure when it fails you don't look like your to blame.
#2 is needed so that both parties take a chance on each other and don't optimize for throwing the other under the bus.