Admin strain. 100% admin strain. That's the biggest reason by far to build new cities, to deal with admin strain.
If you weren't already aware, every asset more than six hexes away from your city but in the same zone adds admin strain. This is offset by assets within those six hexes and the city itself, but if you have a massive sprawling zone that admin strain adds up.
And the thing is, admin strain is a percentage drain of your resources. 25% admin strain? All your assets in the zone produce 25% less stuff. This gets big.
So if you've got some big cluster of resource deposits far in the middle of outback nowhere, set up a new city in the middle of the lot, send enough colonists to work those deposits, and watch your overall resource income increase.
Beyond that, new cities also means that you can build more city-unique buildings like bureaucratic offices, universities, factories, etc. But if you're already short on workers that might not be a huge benefit.
Also note that you can send colonists to pre-existing cities as well, so if you don't seem to be suffering from admin strain but DO have a shortage of workers, funnel folks into the cities.