Tess' death serves a purpose to Joel because she died while trying to fulfil her agreement for the weapons trade between the Fireflies and Robert. Had Ellie's delivery not been a component to her and Joel's weapon trade, she wouldn't have been bit. Her death is also important because it allows Joel and Ellie to flee from the approaching soldiers that Tess distracts as a sacrifice. By dying, Tess also proved that Ellie was in fact immune to the virus. Tess had been bitten within a couple of hours and already started turning while they had seen Ellie last far longer with no side effects. Tess' passing also marks yet another person Joel was close with succumbing to the apocalyptic events that turned his reality upside down. While their departure was a harsh one, it was needed to get Joel on the move. It was easy to see that they didn't want to leave each other and that they were perhaps romantic interests to one another, although it was second fiddle to their emotional bond. The two are realists at heart and knew nothing could be done to change the fact that Tess would be dead soon and Joel and Ellie would be too if they didn't leave her behind.