Corner guard end caps are the caps that go on the top and bottom of corner guards that have an aluminum retainer. Corner guards without an aluminum retainer sit flush on the wall surface and therefore don't need a cap.
Corner guards with an aluminum retainer needs end caps for both aesthetic and practical purposes.
Aesthetically, it is more elegant to have one surface from the cover that curves over the top and meets the wall instead of having a void at the top and bottom between the cover and the wall.
Practically, if there wasn't an end cap there would be a space that dust and debris could accumulate in as well as an exposed edge.
All corner guards with an aluminum retainer come with two end caps and do not need to be sold separately.