Why are caster lords considered better?

Greetings. So i've heard a lot that, when possible, a caster lord should be chosen over the other choices, but I don't understand why.
I want my lord to invest skill points into the red and blue line, not the magic skill tree, and for most races the caster lords don't seem to have much better skills to unlock than a hero. So I fail to see why not go melee lord and caster hero everytime, excluding the legendary caster lords of course.

Thanks