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?

Because growing an organ that we no longer need (human moved away from hardcore vegetarianism to more energy efficient omnivorous diet ages ago) constitute waste, and therefore render the person with appendix evolutionarily unfavourable. Instead the energy can be used for more important things like developing brain cells etc.