A regular axe is symmetrical. A broadaxe isn't. One side flat, the other beveled. This lets you hew a log so one side becomes perfectly flat.
Technique
Log goes on trestles. Carpenter scores every 10 cm. Then hews along the line, chipping between scores. Result: flat wall from a round log. No sawmill.
Why Not a Saw
A saw cuts across grain. Broadaxe — along. Hewn surface is stronger than sawn: fibers aren't cut but smoothed. Rain runs off. Bugs can't penetrate. Hewn barn stands 150 years. Sawn — 80.