The store bought kefir is pricier, more palatable, less probiotics, pasteurized, homogenized, maybe organic, grain fed dairy.
The homemade kefir is easier, more sour/tart, significantly more probiotics. And can be made from many sources of dairy, or non-dairy.
The homemade kefir is easily made sweeter with maple syrup, or fruit, or vanilla, etc.
Pat