Why did mandy moore decide to sing umbrella when it was orignally sung by Rhianna?

It's called a "remake" or a "cover." Artists do it all the time it gains them much needed attention towards their talents by riding the coat tails of an already popular song. Mandy released an album this year called "wild hope," and this probably got her the "shine" she was looking for. Mandy has also done an album of covers called "Coverage." I don't think there's anything wrong with it - Mandy's version of Umbrella is nice, different. Rhianna's version is fun and upbeat. Y! is running a whole series based on covers for example, Natasha Bedingfield did a terrible version of Maroon 5's "This love." LOL. Mandy's remake of "Umbrella" was part of this online series. If Mandy covered the song, she most likely enjoyed it, before choosing it....