Keepers are leather loops that hold the two straps together. Hunting-style sling keepers are for convenience. They are not needed for any function other than to 'keep' the straps together. Order one without keepers if you want the fastest, instantly adjustable leather sling.
For National Match (1907) style slings, keepers are used for cinching the end of the sling around your upper arm to steady the rifle for precision shooting. Very tight keepers are favored for this shooting discipline.
I make Bushcrafts sling keepers tight enough to stay in place once adjusted. It is taken into consideration that over time they can stretch a bit like any natural leather product.
When I first created the original sling design, they were made with all handmade leather keepers. These keepers are held together using only leather and don't use any other fasteners or stitching. They are time-consuming as each keeper is individually hand-cut, formed into shape, and blocked. Blocking the keepers gives them a squared-off shape for a better fit. These are the strongest keepers available.
Recently I started offering brass stapled keepers. They are held together with USA-made brass saddler's staples. These are much less labor-intensive and faster to make. Now I can provide slings at a much lower cost from the time saved in making the all-leather keepers.
Two styles of brass stapled keepers are available on selected slings--A pair of 5/8" wide keepers or one large 1.25" wide keeper. The two 5/8" paired keepers would be good if you like to keep your sling set at a fixed length. If you want the ability to quickly adjust the size of your sling often, go for the single 1.25" large keeper.
Here's a trick to making faster length adjustments with the keeper(s) installed; Slide the keeper(s) down next to the length adjusting braid. Hold the braided knot and keeper(s) together in one hand to slide them all together at once.