[jeffbee]

You can equip this thing with 4 Thunderbolt ports. Each such port is required to provide 15W to connected accessories, and they have to plan for the worst-case current, which inclusive of the greediest CPU and GPU you can configure this diving board with adds up to a huge (worst-case) power draw.

[rektide]

Also, if the user has a dead battery,they want to be able to plug in and charge, ideally while also doing some reasonably heavy computing. So you again need more power than the laptop itself is using.

This is the one case (AFAIK) where input power isn't being converted to some kind of heat: it's being stored in the battery as energy for latter.