These tools do not do the same things. Well they do, but they are for two different applications. Lightroom is a Photographer's tool whereas Photoshop is a Graphic Design tool that can manipulate photographs but to a lesser degree than Lightroom.
Actually, CS3 + Bridge + ACR has become a Photographer's dream...I can't imagine Lightroom doing anything that the Photoshop suite can't.
You are right it started as a graphic design program but it has grown far beyond that.
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