A hook and bar fastening is a popular choice for the closure on tailored trousers, skirts and other garments. It is an alternative to a button closure, particularly where a clean, minimal look is required. It is also a fairly secure fastening that can generally hold more strain than a hook and eye fastening (which is normally just used as a light weight closure where two edges meet, rather than overlap).
The image above shows the position on men's trousers where this closure is most commonly used, as a sturdy way of fastening the waistband of men's or women's tailored trousers. It could also be used in a similar way on skirt waistbands.
The diagram shows how the hook and bar are normally positioned for a waistband that has an extended tab such as on the trousers above.
The other pic shows how the hook and bar can also be used as a minimal closure on another area of the garment as it has been used here to fasten the center front of the cape as well as the adjustable tabs inside the garment.