Distributed state machines are all about deciding in what order things happened in. That A happened before B. If you had a clock that everyone agreed on, much of the problems would go away. The problem is that we don't have one that everyone can agree on. Protocols like Paxos allow us to guarantee an order will be agreed, but it can be slow. They seem to be using the clocks with an error bound to perform a first pass ordering to see if a collision is possible at all, or if they can skip a few steps. Caveat: I don't fully understand that part of the paper yet...