yes can be used to send an affirmative (or negative; e.g. yes n)
response to any command that would otherwise request one, thereby
causing the command to run non-interactively.
The yes command in conjunction with the head command can be used to
generate large volume files for means of testing.
It can also be used to test how well a system handles high loads, as
using yes results in 100% processor usage, for systems with a single
processor (for a multiprocessor system, a process must be run for each
processor). This, for example, can be useful for investigating whether
a system's cooling system will be effective when the processor is
running at 100%.
In 2006, the yes command received publicity for being a means to test
whether or not a user's MacBook is affected by the Intermittent
Shutdown Syndrome. By running the yes command twice via Terminal under
Mac OS X, users were able to max out their computer's CPU, and thus
see if the failure was heat related
via wikipedia: http://en.wikipedia.org/wiki/Yes_(Unix)