Recall that this API 600 requirement is for a wedge gate valve. As the name implies, a wedging force is used to accomplish sealing. To accomplish the minimum seating force for sealing, the disc must be pushed down beyond the point at which the two seating surfaces come in contact. Therefore, the two surfaces are rubbing together and seat material is being removed (wearing is taking place). To make up for this loss of material, the disc must travel  further into the valve to come in contact and seal. The seating takes place higher and higher on the disc seat face.  After enough closings, you run out of seat face, and the surfaces at the top and/or bottom of the seat will not be in contact after the valve is "closed" (little crescent moons).  (In my copy of API 600 there is a figure that illustrates this action.)
The amount of travel required to accommodate this seat wear is called the "wear travel," and the wider the seat, the greater the amount of travel possible. Therefore, the words in API 600 require the manufacturer to have seat widths that will accommodate a specified travel. Since the seat ring is chamfered on the inside and outside diameters, it is the width of the seating face on the disc that controls the amount of wear travel.
During final assembly, the valve disc is fitted to the valve body by grinding the seating surfaces.  It is at this point in the process that the seat ring is verified as contacting the disc seating surface at the design location (closer to the inner edge of the disc seating surface at the stem end of the disc).

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