So that we could sleep for more time. No that wrong.
When we wake up in the morning we yawn and stretch our hands and legs. This coordinated yawning and stretching referred to Pandiculation.
When we sleep we don't perform any activity so our heart rate become slow , blood flow decrease . Also many people lie in same position for long hours their muscle become stiff and tight. Thus a good strech in morning awakens our body , brings our heart rate to normal improving the blood flow to various body part.

Streching also loosens and realigns our stiff muscles making them more flexible and ready for the the day ahead .