Those people you only go to for unconditional and compassionate support. Like baseball keeps some folks "on the bench" until they are needed, bench friends are only used for support. They are not your friends because you don't include them in your friends groups with whom you spend time. You use bench friends, you don't reciprocate with them. You use them because you know they will be supportive and compassionate. This makes the bench friend feel used and marginalized. It sucks to be a bench friend.
"I am tired of being Jane's bench friend; she only calls me when she wants my sympathy."