Paired breakers are also used for "split duplex" outlets, commonly found along kitchen countertops here in Canada.

That's where the top and bottom outlets of a standard 2-outlet receptacle are on different circuits, permitting someone to plug in and use both a toaster and a kettle simultaneously.

The breakers are paired, so that when one switches off the electricity at the (breaker) panel to service the outlet, there is no danger of getting electrocuted by either of the two circuits.

In our new house (under construction), I have spec'd 20A split duplex receptacles for the basement workshop area, so that I can use a shop vacuum at the same time as a power tool, plugging both into the same outlet pair.

Normally, 15A capacity would be adequate, but I spec'd 20A due to a pricing anomaly: the builder doesn't charge extra, so.. why not?

Cheers