Answer:
Good Question.
The purpose of US foreign Aid is to help people in a worse condition than ourselves due to natural disasters, genocide, or unruly governments.
If you mean foreign Aid as in disaster relief, then it is simply to help needy people.
But if you mean the wars on terrorism or the occupations of other countries, then it is the US fighting to defend itself, and in turn helping the citizens of other countries by establishing democracies and dismantling tyrannical rulers.