Why is a Google account required when installing applications from the Market? I installed an application from the Android Market for the first time today however was informed that I need a Google Account. Why is that?
Because the applications that you download are tied to that Google Account.  That way, if you buy any paid applications, and your phone is wiped, or you buy a new phone, all you need to do is make sure that account is logged in on that phone, and you can re-download any of those paid apps without having to pay for them again.

It also enables you to go the Google Play store with your PC, and choose apps to be directly installed onto your phone without even touching your phone.  I do it all the time and it works great.