Because you never know. I was out the other day in the Welsh mountains, off the beaten track up a steep slope and I noticed the mist was coming in. I thought I would check if my compass and whistle were there in the pouch on my belt just in case. They were not, the pouch must have got snagged and had unclipped itself from my belt at some point and in that vegetation I would never be able to find it even if I had known where it departed company with me, I certainly did not find it on the way down so all in all I would have been a bit more secure if I had a backup.