(A bit late to the party, but...)
... but it seems it doesn't really do anything... is it just there for no reason?
Not sure what "it" is...
![Undecided :-\](http://97.107.128.126/smf/Smileys/aaron/undecided.gif)
If you're talking about the pick, a pick is required to do any mining. No pick, no dig.
There has been no research (afaik!) that indicates that a better quality/material pick means "better" mining, either in speed or stones/ores recovered. But, as mentioned about, picks
are legit weapons, and use the Mining skill in combat when equipped (via the
military/
equip/
Weapons menu), which makes a trained Miner w/ a high-quality bronze/iron/steel pick a solid back-up military asset.
If you're talking about the skill, then, as with many "no-quality" labors, Mining speeds up the task considerably. The diff between a no skill starting miner and a +5 skill starting miner is ~about~ doubled, so that miner is now doing ~about~ the work of 2 miners, doubling his dwarfpower. And since it's not uncommon for a starting fortress to have a dedicated full-time miner, that's like having an extra 8th (unskilled) dwarf in your starting crew to help w/ mining - handy.
Mining can be trained up very quickly in sand/soil, and an untrained Miner can hit +5 after a season or so of that, BUT if you need/want to get a fortress up asap then starting w/ a skilled miner solves the problem of underground space, stones and ores that much faster.
(In
my Embark layout, I start w/ a +5 Miner (/Armor User 5), who really gets the first area cleaned out fast. After he gains about +2-3 more levels (Miner 7-8) from the soil layer, I send him to start digging toward the caverns/magma, and I forge a 2nd pick and start a 2nd (unskilled) miner! I put a door/hatch to keep the two separated (so the new miner gets skilled faster concentrating on soil digging), and only open it when the real miner signals that he's thirsty/hungry. Takes a little bit of shuffling of skills on/off to keep them separated, but it's very doable.
Once the main miner can be replaced, he joins the military, usually after a year or so, depending.
Point is, yes - Mining skill can be that important.)