Renewing wedding vows: Why?

In the past year I have heard multiple times of (mainly celebrity) couples renewing their wedding vows.
Mariah Carey has done it a few times, or so I believe I have heard.
Do(es) the first time promise(s) lose their worth after a certain period of time?
Is one half of the couple not so sure anymore about the trustworthiness of the other half?
Is it a trend/hype?
Or maybe it is quite normal and are there legitimate/serious reasons to do so?
What gives?