What Benefits does manager gets by rejecting leaves ?

Hi I work for a client 123 and when ever I ask for any planned leaves before 1-2 months also my manager rejects it . Normally I ask for 2-3 days planned leaves but that also he will reject and incase of urgency he ask me to take sick leaves for those days . Then when to use my casual leaves. Any suggestions ? Or can I take any action if he does not allow me to take leaves . I know he does this for billing purpose .