Powerful offensive spells. Can keep their distance by using lava terrain to their advantage.
Lava Girls and their Hybrid Species all get decent Magic, and work together very well by stacking Burns and utilising rock/lava terrain which most other species struggle with. That's even before taking the Essence and synergy bonuses into account.
Many of their skills can be fused together as well, so even extra Lava Girls can be useful.
Their body temperature is almost too hot to touch, making sex quite an interesting experience.