To print a full size photo on a Canon PRO-4000 (44"x66") at 300ppi, you would need a 261 megapixel image. Now, granted that's not realistic, but 300ppi is generally the gold stand for printing resolution.
A 45mp sensor only gives you an anemic 51ppi when printing that large. So yes, when printing large there is a legitimate reason for having those high resolutions.
Now granted, before people jump all over me with the "but the images still look fine when viewing from a normal distance" argument, yes, I admit that they do look ok. I have printed an uncropped image from my 5DIV at 44"x66", and it does look ok at a normal distance. But really, it's a little lackluster. If you were printing a photo of a city skyline or something with a lot of detail, it would be really neat to be able to get up close and see the small details, but you can't when the initial photo was only 30mp, or even 45mp.