When you’re out to farm for a specific item, you might be doing so to level up your skills, or because you need that item to sell, or to use as an ingredient for something else.
If you don’t remember exactly where to find it, you could of course search for “ITEMNAME farming” on Google, but you can do much better.
Look at those maps, showing you exactly what you need, and you can choose all the items, you want, the area you want, a horde or alliance travel route, names of places and/or areas etc.
The maps uses Google Maps API, so everything you can do on Google Maps, like moving the zone, zooming, etc., you can do here.
You’ll find the maps at http://mapwow.com/
But not only herbs and ores will show up. You can also find treasures on the map, which makes it easy to make a bit of extra gold, when you’re already in the area.