If you want to use the really wide elastic for creating your waistband for boxer shorts or skirts, you'll want to make it an exposed elastic waistband.  Why hide that elastic in a casing when you can make it a part of the design of the garment.  There are different ways to sew on a exposed elastic waistband.  What I like about this method is that there's no raw edges showing on the inside of the garment at the waistline.  A raw edge is not a big deal if your garment is constructed from knit, but you could be dealing with a lot of fraying if your garment is made with woven fabric.  This keeps it simple and hides the raw edge so it'll work with either type of fabric.  Even if your pattern suggests doing an elastic casing, you can substitute with this technique instead.