Why does anyone need anything? The iMac is a stationary computer with a large screen that runs macOS and looks pretty. I don't think most people's primary consideration for desktop vs laptop is port selection; it's whether or not they want the computer to move around with them. If you don't need it to move and will keep it in one place, a desktop makes sense. If you want to cart your computer around, a laptop makes sense.

The iMac is especially nice as a stationary computer for most people: the monitor and computer are integrated into the same housing, so it's one less cable to deal with. With a wireless keyboard and mouse, the only cord coming off your desk is the power cable for the iMac. In comparison a laptop + external display will feel a bit clunky, and will come out to be more expensive anyway. If you never move it, it's a waste.

Because it comes with prestige and they want to show everybody that they have the funding.