So you can tilt both sashes in for easy cleaning. Not a big deal if your house is only one floor but really important if it has 2 or more floors. In addition, if the glass breaks or the seal fails on the top sash, it easy to replace it on a double hung and something you could do yourself in less than 30 seconds. With a single hung, you will likely have to pay a professional to do it for you at a charge.

Why is this thread titled "february"?

That is the primary benefit to a double hung...cleaning and ventilation from the top.

If neither of those are of benefit to you, the Single Hung will usually be a bit tighter and cheaper.