Midnight_v's ResponsePart 1. "Those who slay monsters" I'm not sure what the question is here anymore. The first thing Endarire mentioned was defining the archetype and then slid into why the hell would anyone wanna play "X".
Then a smattering about mechanics here and there buuut... this.
Conan wasn't a caster of any type. But people want to play Conan, because Conan is badass. (though, in all reality, Hyboria is really an e6 world, so people have unrealistic expectations ...).
The whole "why would you want" thing evolves from at least in part from the stories that have historically been popular in fiction and because a maxim for many people playing D&D for a number of years has been something like "What would Drizzt do" I borrowed that statement but I wanted to say its kinda true. There's probably several million Drizzt clones in trash heaps somewhere littering the world. Then there's people who were inspired to make similar characters who are NOT clones. Nonetheless people want to play characters who are embodiments of the things they read.
So they want to do things like "Solo a balor" and "Kill a 1000 orcs" etc because its sounds awesome and that's what people want to be. Awesome.
Most of the time in our history the stories are about fighting against supernatural threats and the most popular stories (or at least the most numerous) end with a swording and not a "Pew pew" wizard power.
There was a quote and I'm butchering it here to be sure but it something like this "Fairytales don't exist to warn children that there are dragons. They exist to teach them that dragons can be slain." I mention this because when someone says "Slay a dragon" the average person envisions a guy with a sword somehow stabbing down into giant reptile head.
Its in essence is a story about "Overcoming" and its a lesson that humans feel we need to learn because of the state of the world we exist on.
Though, people who've played D&D long enough in the age of the internet especially, think about
ways of "killing it from orbit" so to speak.
Which is good, from the perspective a gamer squeaking out ever advantage but kinda sucks from the perspective of telling a tell of heroic adventure.
This is the truth of why people find BATMAN more interesting than superman, (and ironically why lex luthor hates him in someways).
Superman isn't really that brave. He cannot be, because rarely is he in any personal risk.
So "Why" because we love the stories likened unto the heroes of old.
...stay tuned for.... PART 2: "Mechanics!"