Obviously a young'um.
For those of us of a certain age, you didn't just wear diapers -- you wore diapers and rubber pants. In fact, it was the rubber pants that made wearing diapers so embarrassing (if you wore them because of bedwetting or punishment).
I wear plastic panties with my disposable diapers nowadays because there is virtually no side protection with any of the disposables.
Also, since the vast majority of the disposable diapers do not look like any kind of baby diaper I've seen, plastic panties can make you look like you are actually wearing diapers, and not wearing some medical incontinence brief.