Why do we feed puppy food?

I just wondered what benefits puppy food has.
Im working out how much food my 5month old pup needs as im going to feed kibble and wet mix soon, and have found that he will eat up to 2 trays of wet food compared to only 3quarters of a tray as an adult, fed on adult food of course.
Im wondering why such a change???
And at what age have people changed onto adult from puppy food. Last time we had a 6 month old dog who was fed adult food and she was fine! [11years ago now] and shes still going strong.

Hes fed on wainwrights puppy at the moment and 5 months old [15kg] so when do you think he should be changed onto adult foods?

All help is greatly appriciated x

I tend to change my pups over when they have finished growing, I like to keep them on pup untill 18 months old, but depending on the breed you could change over at 12 months or possibly 9 months, I guess each breed differs.