Laying some colors on the appointed sacrificial lamb, so that you can conceal that his entire purpose in the story is to convince readers that people really die -- people with names even -- and he may come to life and make you regret your original choice. Indeed, perhaps he might work better as the Designated Love Interest. . . and you look at detailing the original Love Interest, except that then maybe you have rivals, or a character whose love interest it might be better to provide within this story, or the next.
Part of it is my wimping out on death. But other characters have been known to demand -- at the very least -- subplots. Good for plumping up the story. (I have been known to recommend to NaNoWriMo writers adding a conflict for the minor characters as a way of getting word count.) Not so good for ensuring that the story hangs together.