1. They expel odours.
Bad odours can make using a room unpleasant. From the bathroom, where people go about their daily routine, to the kitchen where the cooking of food takes place, and even the garage where fumes and other pollutants can linger in the air, these spaces sometimes require extra assistance with ventilation, especially if there are no windows. An extractor fan expels unwanted odours, leaving the room feeling and smelling cleaner.
2.They make cleaning easier.
Steam formed by a hot shower in the bathroom, or from cooking in the kitchen, results in tiny droplets of water settling onto cold surfaces such as mirrors and taps. This can often lead to the build-up of mildew, which can be tough to clean. By extracting the moisture out of the space, there is less chance of this happening, making for a quicker, easier cleaning routine. 
3.They help prevent the growth of mould and bacteria.
 Mould and bacteria are health hazards and thrive in damp places with poor ventilation. Extractor fans help prevent this from happening by sucking the moisture out of the air immediately, then mechanically expelling it from the room so that it does not have time to settle and encourage the growth of mould.