Satellites need thrusters for station keeping. Otherwise they drift out of their desired orbits over time.
Attitude control can be achieved to a finite limit using momentum wheels or reaction wheels. But at some point, it will reach its maximum speed and its control capability saturates. You will need to 'desaturate' the wheels and restore its control capability. One method is to produce a counter torque using special reaction control thrusters (RCT) called 'attitude control thrusters'. That needs propellants. Smaller satellites don't have that luxury. So they exploit Earth's magnetic field by using a 'magnetic torquer' to produce the counter torque against it. That needs only power, not propellants.