It's not just the direct damage... Flamewall has additional effects as well:

Enemies entering the wall will be set on fire, as will projectiles which pass through the flames. Many other skills, such as Cyclone Axe, are augmented via interaction with Flamewall.
So ranged attacks may be diverted, and if you pass through the flamewall you'll suffer additional damage for a number of seconds because you're on fire.