Why did Histiaeus tattoo his slave's head? The story is often told that Histiaeus tattooed a secret message on his slave's head, waited for his hair to grow back, then sent him off to Miletus. Why would he have done this?

The story is usually cited as an early historical example of steganography. But a message in someone's memory is less obvious than a message on one's skull. A message in someone's memory doesn't seem more resistant to torture (a strong-willed slave might not disclose the message, while a strong-willed torturer would almost certainly discover the tattoo). It also doesn't seem likely that the message could be kept secret from the messenger (since the messenger could just ask a friend to read it off his skull).

In short, is there some information-theoretic advantage to this anecdotal method?
Exact motives will remain unknown, since we only have Herodotus's word for the whole story, and he doesn't say. However, we can imagine a plausible reason: the whole ordeal provides some level of authentication.

Indeed, Histiaeus's slave could hardly have made himself the tattoo, and certainly not without Histiaeus noticing. Since hair growing is slow (and cannot be accelerated, at least not in Ancient Greek times, where hair grafting was not known), the presence of the message as a tattoo shows at least a purposeful dedication in time to the process. In modern terminology, it is a "proof of work". A contrario, a purely verbal message could be made up on the spot by the slave himself. Aristagoras could believe such an oral transmission to be merely the invention of a disgruntled slave.

Alternatively, this could be Herodotus having absorbed too much Samos wine. He tries to be careful, but since most of what he writes is hearsay, he at times swallows wonderfully extravagant stories, such as some about giant ants that can devour adult camels (one hypothesis is that he confused "giant ants" with marmots, but these peaceful rodents don't attack camels either!).