Just reread this and realized I probably didn't answer the question as you meant to ask it.

If you read close you said GPS is not helpful for safety, but I didn't say it was useless for other purposes.

When you are in a car you are only rarely in a situation where it is even possible for you to be on a different road than what the GPS says. Which is more likely, a car is driving through the fence around my yard, or driving on the road. For navigation it is almost always good enough to assume you are on the one road that it is legal to travel on that is within the error bars. Only in a few cases is there a "frontage road" where there are two options that you can't be sure of which one you are on. In other cases where you cannot be sure you will quickly have moved far enough along the road that the other choice isn't viable anymore.

For lane-level navigation this precision isn't as helpful as you might expect because you can never be sure if your maps are correct. So your navigation systems needs to take other inputs which will sometimes discover the map is incorrect/out of date.

It might be helpful in weather events where the road is completely ice covered - but those are also situations where GPS signals are most likely to be obscured (via cloud cover) and so again you need somethings.

Still if you have more data it can be helpful. However it is only helpful when you can handle it being wrong.

It isn't useful because you need those other things anyway and so it becomes excess data that you don't need. It can still be nice to know which lane you are in, but you have to be able to work 100% without that information, or worse then the information you have is wrong. (that is they just changed the pattern of the intersection and so you no longer can use the second from the left lane to turn)