To spice up a colorless outfit.
When it's cold and rainy (or I woke up late), I have a tendency to reach for mostly black outfits. Colored tights are an easy way to make those bland outfits a tad bit interesting. Not too mention all the comments about your brightly-colored legs are fun.
Even with pants.
No need to give up cropped pants just because it's cold! Wear tights with your pants, and your ankles will thank you.