Because each of the gems on the rosary symbolize something having to do with Mother Mary. Thats all I know. Thats why people say the Rosary at a Funeral.
See, it looks like a necklace because so many of them symbolize something to do with the Virgin mary.
Here's some info here.
http://www-2.cs.cmu.edu/People/spok/catholic/rosary.html
strangely enough alot of symbols are shared between both the pagan and christian religion,(the triqueta for example), and some symbols are shared even between christans and satanist, the upside down cross, allthough i dont really think you can say they are shared as they represent different things but they look the same so....