I recommend that any who rides a tandem have either a multi-tool with a chainbreaker, e.g, Crank Brothers Multi-tool or an honest to goodness mini-chainbreaker like the Park CT-5 AND a reuseable chain link like a Sram Powerlink, Forester Superlink II, or Wipperman Connex.

As for what you'd use it for, basically if you have it you won't need it: consider it a good luck charm. However, if you don't have the tools and parts needed to repair a chain, the probability that a late rear derailleur shift on a steep climb in one of the taller sprockets (28, 32, 34) causing a rear chain to pop a rivet becomes much higher. Seriously, a tandem generates a lot of power at the rear hub and a marginal Shimano one-time-use rivet installation or any other weak link that happens to pass between your rear derailleur's jockey pulley and one of the larger cassette sprockets could pop under a high load.