|P is for Practical|
They don't appear to do anything in any of the apps I use.
Exactly. This leaves it free as a modifier for custom mapping. Under Windows, there are some defaults (Win-L locks the display, Win-D shows the desktop, etc.), and there are third-party apps that let you map it to do more. I use one called WinKey (which isn't made anymore, sadly). When I want my browser up, Win-W. Win-P begins an Eclipse/EPIC session with a blank .pl file. And so on.
In Linux and BSD, they key is assignable to window-manager tasks (as the modifier 'Hyper' in most cases). On my linux devel box, Win-D starts my development environment (Eclipse/EPIC, Devel::ptkdb, and a couple other tools).
On the Mac, it's basically the PC world's "control" key, so almost all apps use it.
But, on any PC-based platform (damn it, I can't say 'x86-based' anymore!), it's one of the most useful keys because you can make it do whatever you want.
ScrollLock, though... what a pointless key. (Keyboard holy war! Yay!)
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