Because often times 3D models UV maps rely on coordinate range from 0..1. If Image Width is 257 then texture's sizeX would be 512. So right side of the texture has black stripe. That obviously cannot be used as a repeating texture, and the usable UV range would be closer to 0-0.5.
edit: Ah it's actually not black stripe. It will clamp with border line like that right side image:
http://i.msdn.microsoft.com/dynimg/IC142381.gif
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