It seems rather redundant, and therefore, inefficient, but this habit may be attributed to the fact that this happens when writing letters. At the end of a letter or a note (or usually, an e-mail), it is customary to sign or write one's name at the bottom.
As for my signature, it is usually a good quote (or two, depending on the site) that fits with my personality. The only time I put my username at the bottom of my posts is when I'm moderating at the forum that I moderate and administrate for. Or occasionally I will do it elsewhere when it is more of a letter (in tone) or if it is something really sincere... or to a small group of members. It really just depends.
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Itaque incipet.
All that glitters is not gold but at least it contains free electrons.