Some homes have no windows in the kitchen. If there is only one flue and you want to install an exhaust fan in the kitchen, many people will connect the duct of the exhaust fan to the flue.
The kitchen is a place for many people who love to cook. It is also a place with a lot of oil fume. In order to prevent the kitchen from having too much oil fume, many people will choose to install a range hood in the kitchen to exhaust the oil fume go with.