That is because a pocket veto cannot be overridden in numerous countries unlike a regular veto. A pocket veto cannot be overridden because it usually happens when there are no sessions of the congress or the parliament. That way a president can make sure that a law doesn't get passed.
Answer: To give congress 10 days to override the veto, To make sure legislation cannot be overridden by congress, to keep legislation from becoming law without having to sign a veto.
