Good question!  Hard to say exactly, but here's one explanation.  Perhaps more apps will take advantage of whatever Silverlight has to offer in the future.

http://en.wikipedia.org/wiki/Microsoft_Silverlight

Need to install?  No, but it is a small download and seems to do no harm.  I've done it with no issues on many a computer. 

when you're at microsoft's website, you may find that in order to proceed and actually view certain pages/presentations there, it will require you to install silverlight first.   if you want to see those particular pages, you may have no choice but to accept it then.

that aside, I don't know of other sites that may use/require silverlight.