Thanks, Askel.
Other than water and a larger lunch (in addition to all the bars and gels on the bike) I was considering the drop bag to be used to replenish the spares on the bike. If I go through some of the tubes I'm carrying, I can re-up on them at the half, same with CO2. I'll have clothes for the conditions on the bike as well, but dry changes in the bag in case it seems warranted at that point.