(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...
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.)