Doctors sometimes request a bone scan, CT or MRI scan before deciding on the best course of treatment. Why is this?

Find/confirm illness. With any imaging method (mri, ultrasound, etc...), the pros/cons must be weighed. What information will the imaging provide to arrive at a diagnosis? Is it cost effective (although patients seldom worry about the cost)? Do i need a live, real-time view (as in ultrasound) or a more detailed view (as in ct/mri)? Is the patient at higher risk of harm from radiation (try to avoid in pregnancy), etc...
Different Results. This scans look at different things and give the physician different information. The bone scan looks at bone metabolism. The ct and MRI look at bone anatomy. Ct is better at imaging certain parts of the body than MRI and vice versa.