There are plenty of legal things that are difficult to purchase with conventional reversible electronic payments. Off the top of my head:

Gambling deposits (yes, these are legal in most of the world but plagued by chargebacks from losing punters)

porn/sex toys (legal, but people don't want it showing up on their CC statements and don't trust these sites with their CC number)

"Suspicious" purchases with too much chargeback risk (eg. someone wants to buy a Macbook online with a US credit card and a Nigerian shipping address)

"Cash-like purchases" like buying a gold bar or some foreign cash online and having it shipped to your house. The margins on these types of purchases are too small to cover the credit card fees and the chargeback risk is too high because it attracts carders.

I could probably go on, but if you don't think irreversible electronic cash has any legitimate applications, you're not thinking hard enough.