Why is the president allowed to appropriate government money in times of a disaster

The correct answer is: because he can react to emergencies more quickly than Congress.

The president can appropriate government money in times of a disaster, to react to emergencies quicker than the Congress.

This situation only happens in times of emergency, in which the President is allowed to override the Congress and deliver executive orders without the Congress permission.

These emergencies are declared when: there is an armed conflict, a natural disaster, civil unrest, etc.