You can have the flexibility of using a drawstring to tighten the waistband while not worrying about your pants falling completely down when undoing the drawstring. This is most commonly seen in pajama bottoms and workout pants.  While the technique is similar to doing an elastic waistband, there are some difference. This video demonstrates how to easily convert an elastic waistband into a drawstring one, using buttonholes for the entry of the drawstring. What seems complicated is actually fairly simple to achieve and looks really nice.