Some Garmin devices can receive traffic data via radio, which your phone can't do. Garmin also usually has business names and gas stations, whereas I've found Google maps offline will only let you navigate to a specific address (Android), and it's reliant on you remembering to store the map offline. If you found yourself in an area without signal, and you didn't pre-save, you'd be screwed, which I've had happen plenty of times.

Personally I use offline maps, but only on specific trips where I know for sure I'll have no data

It’s one of those always be prepared things - I was making a routine trip through Albuquerque once where just outside of Tucumcari I encountered a traffic jam from a huge crash up involving many vehicles and fatalities that ended up closing both directions of the interstate for ten hours. I managed to get off onto a farm road and found path around the accident - fifty or sixty miles of two and one lane farm roads, barely populated, at dusk. Passed more rabbits then cars. Twice I had to get out and check what was on the other side of the hill I was about to drive over. But turns out they were real roads and I made it To Albuquerque in time for a really late dinner at Waffle House. The interstate opened back up about six hours later. Would I ever have attempted that with only a phone? Never. If my route got cleared for any reason I’d have been in a bad situation. I don’t trust a phone app would have downloaded enough information to even compute an epic detour like that - I went well over thirty miles away from the original route before reaching the road that went back.