It's just for conveniences issues.
For examples, let's consider I have a sprite with 9 different textures, let's called them mytexture-001.png, mytexture-002.png, ..., mytexture-009.png
One of them could be just transparent, but it's just much more simple for me to simple replace packs of textures (even if some layout has one transparent portion) than replacing UISprite by invisible widgets.
Anyway, in case I just want to do it: how should I do it? Or am I just not able?