There hasn't been much discussion about lash caps over the years on this forum. I have always used lash caps in my 289. Originally I believe it was because it helped prevent the ends of the stems from becoming worn from the roller tips. They fit on the end of the valve stem, the rocker arm roller tip contacts the lash cap. There is a minute hole in the cap to allow
oil in & out from between it and the valve stem end. I am using Ferrera valves throughout. I recall that I had to have all the ends of the lash caps that fits over the stem surface ground to reduce their length a few thousandths, so they would not contact the valve locks or keepers, so far no problems.