Yes, it is very useful as you can map when you have no mobile data or it is spotty.

At least in my area you often have highly degraded connectivity while seeing city center. For example forest park at https://www.openstreetmap.org/#map=18/50.060283/19.845654

To say nothing about more rural areas or forests outside cities.

Also, it allows loading maps on wifi and editing as tourist without paying bajllion/KB roaming fees.