Why do people "resing" a hit song?

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Why do you think stars will pick a song that is already a hit and resing it, especially if they resing it without any real changes? I can understand if they choose it to resing with their own style or something but to simply repeat the song with the same basics of the original make no sense to me. A perfect example is Pink singing "Bobby Maghee". She's singing it on the Today Show and she sings it exactly like the original. Now if I wanted to hear the song and loved it I'd listen to it as sung by the original artist, not a mimick of that artist's style sung by Pink. Wouldn't you? I love Pink and love HER style.. but this obviously is a song she's singing in someone else's style. I'm not sure what the concept is behind it?