Why does David Letterman have a beard? To put it more precisely: Why did Mr. Letterman, after stepping down from CBS's "Late Show" and a 33-year career in late-night television and all but disappearing from public life, spend the past year and a half cultivating a fleecy and prodigious mound of facial hair, which, depending on your disposition, makes this 69-year-old entertainer look like either a lanky Santa Claus or an escapee from an asylum?