You didn't design the phone. You designed a rectangle inside it, and that rectangle has to carry the whole job.
Borrowed hardware does something no amount of presenting does. A screen on a white page reads as a proposal. The same screen inside a laptop reads as a thing that already shipped, and the conversation quietly moves from whether it's finished to whether it's right. The machine is a costume, and it works. It also carries a date — put your work in a handset from a few generations back and the work ages with the hardware, whether or not anything in it is old.
The shape is not up for discussion. You take the proportions the manufacturer picked, the corners are rounded whether your layout agrees or not, and there's a bite taken out of the top of the phone sitting exactly where you wanted the title. A laptop is hinged, so its screen never quite faces anyone straight on. And the whole thing shrinks in a deck — type goes first, and what's left is shape, contrast and colour. If that turns to grey mush, nobody gets as far as your layout.
Devices are where finished work goes to be looked at: pitch decks, portfolio cases, store listings, landing page heroes, the slide the client actually remembers. The machines vary more than the work inside them — phones, laptops, tablets, watches, the odd monitor — and something that holds on a wide screen collapses into a stripe on a small one. The words drift too. An American cell phone is a mobile in Britain, laptop and notebook depend on which trade taught you, and marketing calls the finished picture a hero shot while you're still calling it a mockup.