Protein shakes are just easier for people who don't like consuming so much meat/eggs.
They are good for both weights and cardio, they help you go on further and they definitely help with muscle recovery.
If you are doing a few presses at home its not worth it, for the amount of calories, but you'll find most of the people here on protein shakes are actually members and regular attenders of a gym =P Its not worth spending the money if you don't use it properly, and all of us know that!
For the people trying to lose weight I found an interesting resource: http://www.dukandiet.co.uk
Check the "True Weight" test. Mine is 13 stone 7, which puts me at an "overweight" BMI of 27!!! Hard to believe! Good for people who aren't sure if their bmi is an accurate representation of their body.