Well I know there wouldn't be much to do other than socialize, waiting for drive group, car show meets, and explore as it's not GTA (no I don't want casino back), but the exploration would become way easier in beautiful locations. Not having to try navigate with a big car in close quarters and allow areas inaccessible to cars is a good thing. (not like anyone? would go walking along highway or random woods). I also miss walking at all the other events to look/while waiting.
I KNOW.. Photo-Mode can do wonders but it's not.. what's the buzzword... IMMERSIVE. ;)
Anyway it's not like compulsory to do or hard to implement as they've basically already done it. (wasn't there even a mod that enabled it for open world or was it a glitch?)
Plus you finally get to use and hear the actually rigorously designed sound of already functioning car doors more. (never checked much but the sounds better be correct and not crapo-generic ^^).