agree with the people that say you are moving the goal posts, but to answer this question anyway...
As someone who lifted for a good handful of years, there are a few reasons i used protein powder, it was a very affordable way to add 25-50g of protein and some random fruits or peanut butter or whatever(i'd usually blend up a shake).
It was also a good way as someone who struggles to eat a surplus, to hit my goals as it just went down way easier than an additional full meal.
It is ALSO easier to cut weight and maintain protein goals by utilizing simply water and protein powder.
when it came time for me to cut, im simply swapping milk for water, and removing the peanut butter, and suddenly that "meal" is ~400 calories less.
So the very simple answer? convenience/affordability.