Simple.
1. To carry out the foreign policy objectives of US/NATO and their Globalist Bankster puppetmasters when conventional forces cannot be used. The American people are broke and weary of war and would be upset if the US military got involved in yet another immoral, illegal Mid East quagmire. The use of Al Qaeda mercenaries is much cheaper and much easier to hide from the public - as if they would care even if it were publicly known.
2. To use the threat of Al Qaeda boogiemen to scare the American people into giving up their liberty for a little bit of (false) security.
Remember, "Empire abroad - tyranny at home."
You might wish to read American History FRiend.