What is the purpose of reserving an upgrade of a piece of software? It's not like physical objects where there's some limited quantity, they can just keep on making copies.
Well reserving it now would mean that in the event of a pricing change or something like that, you'd still get it. At least I couldn't imagine any other use for it.
"[why they do it?] So that they can get developers on board by saying "x million people have reserved their upgrade to Windows 10, so get your apps ready!"
Also to give people time to address any compatibility issues (the application runs a compatibility check.)"
It's a free upgrade, so why not? Those that aren't eligible to reserve a copy of Windows 10 would have to pay around $130 dollars if they wish to update, or so I've heard, and I suppose I'm lucky that I'm not one of them, haha. From what I've seen so far, Windows 10 is looking pretty decent, and I'll be installing it as soon as it's released.