we all know it but can never be remined too many times:

Float Clearing

One more popular uses of setting overflow, strangely enough, is float clearing. Setting overflow doesn't clear the float at the element, it self-clears. This means that the element with overflow applied (auto or hidden), will extend as large as it needs to encompass child elements inside that are floated (instead of collapsing), assuming that the height isn't declared.

http://css-tricks.com/the-css-overflow-property/