Dragons only do a few things, but they do those things very, very well. Dealing damage is one of those things.
A dragon's normal attack is used in both melee and at range, it never misses anything it can touch, its damage can't be resisted by any creature, and it gets stronger with more points in the Dragon skill. It does have trouble hitting creatures that are very low to the ground (like rats), it can't hit enemies through doors (normal weapons can hit an enemy through a crack in a door, if you can catch a glimpse of the monster's sprite), and it may have some issues connecting through narrow gaps or edging past corners. Also, the damage of a single strike is highly variable.
Dragons also bring fireballs, flight, loads of hp, and GM Id Item. And that's pretty much it.
They aren't useful for healing, and their only utility spell is flight, so you'll need a cleric and necrmancer to cover your healing, protections, portals, and enchanting.
You'll find that a late game Lich can reliably destroy monsters with Dragon's Breath, but said monsters can't be close to the party or the Lich will flatten your party too. Give the Lich time to build up to 30's or 40's in Dark Magic. In the meantime, the dragon will just keep mowing down monsters.