Deployment. Gas Saved: 24 227
Minimum Method Call. Gas Saved: 254
Average Method Call. Gas Saved: 533
Maximum Method Call. Gas Saved: -6 726
Overall gas change: -1 371 (20.741%)
Saves a storage slot for the mapping. Depending on the circumstances and sizes of types, can avoid a Gsset (20000 gas) per mapping combined. Reads and subsequent writes can also be cheaper when a function requires both values and they both fit in the same storage slot. Finally, if both fields are accessed in the same function, can save ~42 gas per access due to not having to recalculate the keys keccak256 hash (Gkeccak256 - 30 gas) and that calculations associated stack operations.
