Remote sensing is observing and measuring our environment from a distance. So, remote sensing satellites are usually put into space to monitor resources important for humans. For example, remote sensing satellites might track animal migration, locate mineral deposits, watch agricultural crops for weather damage, or see how fast the forests are being cut down.
All of these things can be done best from space because a satellite in orbit can normally take photographs of large expanses of land all over the world. Since these satellites are able to take photographs and observe areas all over the globe, the satellite is able to monitor areas in which the climate is very harsh, or which are nearly impossible tor reach by land.
Canada's main remote sensing satellite, Radarsat, does just that.