I think this would be beneficial for many reasons.
1) Your more likely to agree with the politics of your county than the politics of your state. This means I am more likely to find politicians that agree with me on most things.
2) It allows for counties to be themselves instead of being bossed around by a state. It allows upstate NY to be more conservative and it allows for Austin to be more liberal. Currently, Austin has much conservative influence due to being in Texas and Upstate NY has a lot of liberal influence due to NYC. Letting every county run themselves to an extent lets Austin be as liberal as they want and it allows upstate NY to be as conservative as they want, provided both of their ideas are in line with the constitution and supreme court cases.
3) Other counties have more autonomy and you have even more experiments within certain counties. Currently America has 50 states with 50 experiments. If every county was given the same autonomy a state currently has, we have up to 3143 experiments running.
4) It makes it easier to move to a place that you are more ideologically in line with. A liberal person from Alabama has to travel hundreds of miles to be in a blue state. Now they just need to move to the nearest liberal county which will be much closer. A conservative from New York doesn't have to go all the way to Texas to be somewhere conservative. They just need to find a right wing suburb if they want to be in a conservative environment.