I'm not married, but the only time when I would see there being a point to renewing your vows is if they had been broken, the marriage had been strained, and they want to re-comit to everything they promised.
I don't see any need to renew wedding vows. A vow is a solemn promise and it has either been kept or it has not. Marriage is a continous state of being while that marriage prevails and so are vows not then continuosly renewed with each passig moment the couple is together. I would not renew my wedding vows because I meant them so sincerley when I made them and I meant them to be for life.