My first reaction is just why? under normal circumstances.
I'm sure this practice didn't exist a few years ago, when everyone was a lot less showy and stoicism was a virtue. I perceive this renewal of vows palaver in some ways to be c'leb driven. Make a quick buck by getting it covered by "Hello" or the like, all part of the whole gamut of putting "so called private lives" in the public domain. Starting with big flashy wedding, childbirth, weight loss and boot camp, then my nightmare hell, any combination of cheating partner/cocaine meltdown/failed business venture/bankruptcy/breakdown/rehab for PTSD, Recover from all of that by renewing vows before the inevitable divorce. On to the next marriage and repeat sequence, making sure it's all fully covered by trashy magazine or tv fly on the wall, whilst simultaneously asking public to respect privacy.
However, I can see, as outlined by LullyDully above, the desire to mark surviving something traumatic in a special way is understandable.