Closed-cell foam backer rods are used behind chinking and caulking. Foam backer rod is essential for ideal joint design. The rods are used to provide backing for 2 point adhesion to sealants such as caulk or thinking.
A thin layer of sealant stretches much better than a big ol' wad does. If it can't stretch, it'll either pull away from the log or pull apart, breaking the seal and increasing energy ($) loss.