How is losing an ability part of evolution?

Alright, so it's believed the appendix and one point helped in our disgestion of dietary fibers, and we could consume grass/bark/plants like heribovores. So how is losing that ability evolutionary?

>>11
No, I mean actually digest it. Humans cannot digest dietary fiber. It just passes through their systems. Other animals, like cows, can actually get enough nutrients out of grass and hay to grow, mature, and eventually bear children. A human could not live on grass.