Tablet cases with handles are designed to serve as individual pieces of luggage. They don't need not be carried in a bag. Some brands even sell handles for existing cases as separate pieces.
Cases with handles come in many forms. Some portfolio cases and even smaller hard cases may have this feature. Some cases have handles sewn into the leather exterior and, like any sort of luggage, will either be comprised of two independent pieces or two handles which may be fastened together to form a single handle.
The advantages to tablet cases with handles become obvious during travel. As a distinct, lightweight unit, cases with handles make it easy to carry them around in a hurry and without dropping.