Much like the loop on the back of a tie, this useful little loop above the trouser fly is designed to keep things in place, viz. your belt buckle at the centre of your waistband.
A pair of trousers will usually have six to eight belt loops, and the greater the number of loops, the more contact with the belt, and hence less slippage. Because the distance between the two frontmost beltloops is greater than elsewhere on the waistband, the belt tends to slacken, slip, move up or down, and generally misbehave in this most important of regions, especially in the case of a pair of trousers with rather more room than is ideal.
Enter the prong keeper.
Into this loop goes the buckle’s prong, and the belt is fixed in place, impervious to twisting, bending, seating and rising—possibly even dancing.
Its length should be slightly greater than half the width of the waistband. Trousers designed with care usually feature an upward-looking prong keeper, the addition of which to a pair of trousers requires extra steps while sewing. It’s much easier to make a loop that looks downwards, but this invariably results in the creation of unwanted tension in the area and puckering of the waistband. Mark it: an upward-looking prong keeper is a sign of special care having been taken in the construction of your trousers. This can be taken to another level: the absence of visible machine stitching on the prong keeper means especial attention was paid to finishing the keeper in such a way that any stitching remains on the inside, unable to be caught or pestered by repeated introductions of prong to keeper.
Additional function may be wrought from using the prong keeper as an aid in zipping your fly, should extra muscle be required. All in all, a helpful and worthwhile feature of well-made trousers.