Vehicles don't care for encumberment and (aside from the bicycle) don't drain your stamina (both factors are especially important, if you are out in the night). As you already mentioned, they also bring storage with them. Just put your non stackable and/or rare stuff in them and keep common stackable stuff in your inventory. You can easily loot 2-4 times (depending on the vehicle) as many buildings in one run, instead of walking back after each building or even multiple times per building. That's a huge timesaver. It would even be a timesaver, if vehicles were a bit slower than walking.
A19 has vehicle mods planned, the current vehicles have the slots for mods but none are currently in the game. I can imagine a few types: Gas tank, Speed, Hill climbing ability, increased storage space, maybe a armor mod to give them more health.. not that they need it really but there just to be there.IF that's going to be the logic then there should be a type of vehicle with no storage that is significantly faster then walking. But the slow speeds combined with the ability to not be able to navigate slopes often makes vehicles far slower then just drinking a beer and running.
Now if there are mod's like a turbo charger that increase speed, performance tirers to let them go up hills better, etc then its a different story.