But using Agile allows you to show this in concrete terms. Waterfall uses the "man hours/months/years" metric which is useless given the difference in productivity between developers. Agile, on the other hand, shows the output that is occurring on average, i.e. what you're realistically going to get.
What I tell people is: what do you want? Do you want to never be told no, but never get what you were promised? Then use waterfall. Do you want to usually get what was promised, but be told you can't have what ever you want? Then use Scrum.
What I tell people is: what do you want? Do you want to never be told no, but never get what you were promised? Then use waterfall. Do you want to usually get what was promised, but be told you can't have what ever you want? Then use Scrum.