iserith
Magic Wordsmith
You got it. There is a good reason to return to town. Or if you're a ranger or a druid to return to your shack in the woods/magical grove/whatever. Encourages the party to have a home base, which is good for hub-and-spoke adventure design.
In a hexcrawl game I joined, the DM wanted the characters to have incentives to go back to town so we worked out a house rule that hit die recovery only happened in a "safe haven," which is a settlement or any special place the DM designated. So there might be "safe havens" hidden in a hex somewhere which provides a nice reward for exploration. It also encourages the players to have their characters create safe havens, a decent gold-sink.
The game's just started, so we'll see how it works out in practice.