Why do doctors put a cast on a arm or leg?

This is to hold the bones in place in the right position for them to heal properly. If this isn't done, they may not match up properly and result in loss of mobility and function. Increasingly, plaster casts are being replaced in some instances with air-filled casts that use air pressure to hold the bones in place.

The surgeon re-sets your bones back to the correct position, so they can heal. The plaster cast holds the bones in place so they can 'knit' back together - and, so you can use the leg (with limited movement) while the healing takes place.