The Nilometer allows Egypt and Nubia to build large, productive cities along the banks of the Nile River. The Science bonus is nice, and the extra Food and Production from nearby Floodplains make this terrain much more desirable to a city with a Nilometer.
The people of the Nile Valley depended upon its annual floods (or “inundations”) for their very survival. The ingenious nilometers measured the height of the floodwaters: too high, and people could expect dangerous floods. Too low, and they would see famine. Anything between those marks indicated a relatively prosperous season.