I would much rather eat a steak or have cottage cheese over a protein shake. Any day of the week, all day.
But I can't with my schedule. I can't sit down to eat a can of tuna, or prepare a steak at any hour of the day.
So for me having protein shakes are convenience, necessity, and possibly most importantly an insurance or peace of mind for me that I'm getting something rather than missing a meal altogether.
The theory that I have, forgive the example, is Greg Kovacs in his early MMI "articles/ads..."
"I would rather skip a workout than a meal. With a workout you can make it up at a later date. When you skip a meal you have to start the process over again and that can hinder your progress."