What are the benefits from each additional point of population?

Solution 1:

Citizens, the products of population growth, are producers as well as consumers. Each Citizen in a city produces 0.7 Science and 0.3 Culture per turn and can work one tile within the city's borders, adding its yields to the city. Citizens can also work as Specialists, filling slots in buildings in specialty districts and providing the city with additional Culture, Science, Faith, Gold, and/or Production based on their role.

(http://civilization.wikia.com/wiki/Population_(Civ6))

There are also two Civics boosts that require a 10 and later 15 pop city to trigger.