Why do cities in the US advertise their population size on their signs? What purpose does or did it serve?

by flooopthepigg
rm999

Old thread, but not too much in there. u/MrDowntown gave an answer but it's unsourced.

https://www.reddit.com/r/AskHistorians/comments/3dk7tr/why_are_population_numbers_generally_listed_on/