A tapping fixture is good for tapping small holes in lightweight or thin material. It helps to have the tap held absolutely vertical, and to have the material being tapped securely-fixed in place. That will give you high accuracy with your threaded hole.

With a deep hole, you just fix the tapped piece in a vise, and the long hole will keep the tap properly centered. With thin matierial, though, a tapping fixture is desirable because it keeps the proper alignment.