[bastawhiz]

If the speed of yes is bounded by memory speed, doing anything useful will almost certainly consume that data at a far lower rate. Putting it on a disk, pushing it over a network, etc. will almost always be slower than yes is able to generate data.

[cestith]

The typical use case is piping it into another running program. Maybe someone wants to do that really quickly rather than putting it on disk or pushing it over a network.