This really matters in map apps when you're on the road. Apple Maps is all very nice with transit planning but without a solid net connection it lies there mutely. Organic Maps (and others) that use downloaded OSM data work well with nothing more than GPS. You can get a transit plan (trains & trams & buses, and the transfers among them) without up-to-the-minute arrival info.
Google and Apple maps have an offline mode. Just select the area you want downloaded and that's that.
Last time I tried with Google I could only download a relatively small area, and it would only be available temporarily.
With CoMaps, I can download entire countries and keep them forever. That's pretty different.