This is yet another example of Christians being influenced by man's theories outside the Bible. As the Bible is a revelation from the One who is infinite in knowledge and wisdom, then all of our thinking in every area should start with God's Word. Let's assume that modern science knew of no function for the appendix. Not at all. Thus, our body has lost some functions which it once had.
Evolution requires a gain in information new structures, new functions.
It has a created function, but we don't know what it is yet. This is not what we would expect from a degenerate, useless structure. The appendix contains a high concentration of lymphoid follicles. These are highly specialised structures which are a part of the immune system. The clue to the appendix's function is found in its strategic position right where the small bowel meets the large bowel or colon.