The messages badge signifies the number of voicemails. This badge is removed by appropriately handling the voicemail. Without this badge one would not know they have a new voicemail message.
From a UI/UX standpoint, until you pointed that out, it was completely unclear (given that I have a # of "messages", aka "calls") that this was the case. The VM in question was mid-way down a list of about 50 items, not highlighted in any particular way (it does have a different icon but that was easily overlooked).