In WOW you can have up to two secondary professions (alchemy, blacksmithing, herbalism, mining, enchanting, engineering, jewelcrafting, tailoring, skinning, leatherworking). You don't have to do this ... your level 60 warrior with blacksmithing isn't any weaker than a level 60 warrior who never took a profession.
Herbalism, mining, and skinning allow you to acquire extra resources (herbs, ore, and skins). You use these resources with certain crafting professions -- herbs for alchemy, ore for blacksmithing, engineering, and jewelcrafting, etc. In some cases the items you can make are common but useful things (like simple heal pots, or sharpening stones that give a temporary bonus to weapon damage, or armor kits that add to an armor's protection value). Sometimes they're really good (usually requiring some uncommon ingredients but you can produce some nice things to wear, like a fire protection breastplate or whatever). Even if you can't use the crafted item, you can sell or give it to another player. Even if you just have a gathering profession (herb, mine, skin) that means you're getting extra resources which you can sell, and thus have more money to afford cool stuff you want. And it's fast (because taking an hour to craft a helm may be unrealistic, but you don't want to sit there logged in doing nothing for an hour). No chance of failure, either (it would suck to spend an hour playing to find a rare herb and have your potion-making go wrong). And there are some really awesome crafted items that you can't get any other way -- a player has to make them.
Compare to D&D's mundane crafting. No description in-game how to acquire raw materials, which means you default to buying them. An abstract method for creating mundane items (like armor) ... which you can then sell at half price. Because it's not as good as the stuff you got from the ogre you killed. After weeks of work. And possibly failing some Craft rolls. And possibly wasting materials. Basically not profitable or useful to your character, just a roleplaying thing, or a way to turn small money into slightly larger money over weeks of in-game time.
Compare to D&D's magical crafting. Item creation is still pretty slow, pretty expensive, abstracted cost (which isn't necessarily a bad thing), problematic in determining cost, often inferior to stuff you can loot from dead monsters.
It would be really cool to see someone do a good port of WOW's item crafting into a D&D setup. You'd need to give characters free specialized skill points (perhaps 1 per level that could only be used for this so you wouldn't be sacrificing other adventure-worthy skills) to put into these harvesting/crafting skills, and give them a set list of recipes from trainers (and rare ones from monsters, just like WOW), and let people actually make stuff they'd want to use. It would take a lot of work but if done right it would be very, very awesome.
(Yes, I know that D&D is the way it is because you're not supposed to be sitting in town forging swords, you're supposed to be out killing monsters. But if done right it would be an interesting addition to the game that wouldn't detract from the main elements of D&D.)
(As an aside, I wanted to put some cool gatherable resources in parts of Expedition to Undermountain where you could bring them back to NPCs to get minor buffing items or even the recipes to make those items but I got cramped for space and time and I don't think I wrote any of it down in a useable fashion.)