It would be nice, in the case where... say... a parent allows their child to use their iPad for whatever reason (productivity or gaming). It doesn't necessarily make sense to spend hundreds of dollars on a separate iPad for the kiddo(s).
And even in the event you did purchase a cheap, refurb iPad specifically for your child... what if you had two or more children? Having 3+ iPads floating about the house is not exactly an elegant solution, either. Plus, if you cared to lock down some of the iPad's apps or functions, restricting them to certain users or only allowing access at certain times of the day... etc, etc. Your usual parental controls stuff.