the logic of using a passphrase when generating a key

[Christopher R. Jones]

I've found documentation on how to create keys with ssh that don't say
anything at all about using the passphrase option when running
'ssh-keygen'.  Then I also find documentation that only shows you how
to  create keys using a passphrase when running 'ssh-keygen'.

My question is... what are the pros/cons of using/not using a
passphrase?

Thanks,

-chris