Well, OMNY only charges you for 12 rides a week (with all the following being free), but only if you use the same card/device to tap in each time. The way an OMNY card works is that you can reload it online with any credit or debit card (and eventually you'll be able to reload it at vending machines, presumably with cash). So I guess if you want to use multiple cards/accounts to pay for transit you can use the OMNY card to make sure that you still get the post-12-ride discount.

And for families... I can see parents setting up an OMNY account for their kid, giving the kid a card and pre-loading it with cash, and that way the kid has a way to get around.

Also, if you have a non-contactless transit benefits card (like me 😔) then you can use an OMNY card to spend that money.

Lots of reasons.