Let me "paint" a picture for you. So you just purchased your home and while doing your inaugural tour of your new place you notice some used paint cans sitting in the basement. You do the dance of joy because of the previous owners generosity. What a great closing gift, now you have FREE paint to touch up things.

What you may not realize is that some paint can be a hassle to dispose of. For instance, oil based paints are considered hazardous waste and cannot be thrown away in the trash. To properly dispose of oil based paints you would need to take them to a Hazardous Waste Facility. Find a local Hazardous waste facility here.
Unlike oil based paints, latex based paint can be thrown away but its not as simple as you would think. The process involves making sure all the paint inside is dry which could take days. To speed the drying process up you can always add an absorbent material such as sawdust or kitty litter but still can be a big inconvenience...especially if you don't have a cat or wood to cut :)

Next time you buy a home look for used paint cans during your final walk through and make sure you are comfortable taking on the project of disposing of them properly. If not, ask the seller to take on this responsibility. You will save yourself from a great inconvenience.

Here are some helpful links:

Find a place to properly dispose of your paint

Paint and Paint Can Recycling 101