There are a couple of reasons but the main one I've noticed is that you might have someone (or a few people) on your list that has a FB account used for spamming, catfishing, or trolling. You announce that you're cleaning your friend list to make sure you dont delete friends who are 1) legit people and 2) actually want to keep in touch. You might also have friends with more than one FB account, so you want to make sure you don't delete one from your list because it's inactive.