I have lived with dogs all my life - when I was born our family dog was already 1 year old! As far as I can remember, we have always put a small amount of gravy or stock on our dogs' meals. My elderly galga Carmela now has problems crunching large croquettes so she is on the ones for small dogs which she manages no problem. However, I must admit I am not keen on the idea of soaking croquettes for hours.
Here's another interesting article from the Whole Dog Journal on the subject.