The easy answer is: You can get closer to the subject of your photographic desires. Focussing closer means getting the subject larger on your sensor.
So you can have 50/60/90/105 even 200mm macro lenses and they have only one thing in common. They focus closer than their standard counterparts.
Whether you need a macro lens or not is easily tested. Just zoom out your lens to 135mm and focus manually to the closest distance. Than look through the viewfinder and approach the object of your desire (e.g. a page from a newspaper or magazine) until it is in sharp focus. If the object is large enough for your taste - fine, you don't need a macro lens. If it's to small still you need something stronger than your current lens.
More information can be found here http://www.ephotozine.com/techniques/in ... ?subject=3
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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