Well, we'd stop having to spend so much taxes on redistributive efforts, again, like subsidized internet. It's up to voters and politicians to actually change the tax rate to save the money. It'd reduce government debt at least.
Electricity would be cheaper. Here in California, a significant amount of the (very high) electricity costs are used to maintain rural power lines. If rural people moved away, we'd be able to decommission them and no longer maintain the lines.
It wouldn't happen immediately, but as more people become urbanites, we'd be able to move gas subsidies and government road maintenance spending to the urban environment, where we'd spend on more drivers-per-mile roads, OR shift to public transit funding, or simply reduce that government spending.
Over time, we'd be less reliant on cars, which reduces everyones costs, but will mean we aren't so desperate to protect oil interests, so we'd be able to stop paying for wars in the middle-east. Honestly this alone has so many positive side-affects it'd be hard to actually enumerate.
