Good question and in some cases I am sure that it may be true that people are trying to recreate something else.
However, I think it's like cooking. You know how when cooking you taste something and think, "Hmm, that could do with a bit of "X" to make it taste better"? Well I think it's like that with layering.
For example I often layer Gucci Homme with Obsession, because I feel the Gucci needs a sweeter top to it and Obsession does the job beautifully.
I also layer a base of L'Occitane's Ambre with L'Iparie over the top, which ends up a beautiful and unique combination.