When scaling an object up or down, its stroke may end up looking too spindly or too thick in relation to the object if Scale Strokes is turned off. Conversely, you may not want the stroke weight to change as you scale an object.
When laying out a newspaper front page, for example, you may wind up resizing photos multiple times, but your style guide still requires only a .5 point stroke around photos. Resizing the photo then using the Stroke palette to fix the line around it is ineffecient (and everyone knows I hate inefficiency).
Whichever your gripe, strokes scale when the shouldn’t or don’t when they should, it’s an easy fix. Just toggle the Scale Strokes menu command from the Transform palette flyout menu.