As water drains through your container it will sit underneath the bottom of the pot and sometime this could cause discoloration of the surface that the container has been placed on such as a deck or a patio. So in order to prevent this you can either use a saucer such as this one, this will actually collect the water. The one disadvantage to this is if you have very large plants you need to empty the water out after you water your plants, otherwise this can become a breeding ground for mosquito's. So another choice that you have for a very large container would be something called pot feet. So these are just little clayed blocks that sit underneath the container and elevated just enough so the air can circulate underneath the bottom of the container.