When you see chairs lined up either side of the road, it means tonight is the Carnaval parade in Atlántida. But they’re not there just as a nice gesture. If you want to sit, you have to pay.
I went out to see part of the parade one year, and haven’t felt compelled to do so again. You can find more in the Wikipedia article on Uruguayan Carnival (which apparently doesn’t meet Wikipedia’s high editorial standards, oh my!) and find videos of our local desfile here.