Empty butter tubs can have a variety of uses. i use them to store food (instead of tuperware) and hold my ingredients prior to cooking. i don't normally measure my ingredients but since a regular tub is equivalent to a cup (of course, you already knew that!), you may also use these to measure or at least estimate the amount of whatever you're using.
we also recycle them to pack away loose items like paper clips, buttons and even extra ketchup sachets. i just love how my toddler's crayons fit perfectly inside this little tub of butter!