The popularity of dying cigars came about during the cigar boom in the 90's. Manufacturers noticed lots of people were asking for darker sticks. Quality doesn't have a lot to do with wrapper color per se. Dark leaf is a result of the aging process and temps the tabacco is cured at. It is a longer process to darken a leaf, therefore more expensive to use, since it requires building a bit of inventory. Some folks feel, mistakenly in my opinion, that a darker leaf tastes better. This just points to the fact that taste is subjective and there is a market for everyone.
The idea of wetting ones cigar comes from Mi Ron Nee who some hold as the guru of the cigar world. Me, not so much. He does have a rather large collection of aged smokes. Being a doctor I suppose, he has the income for such. Nee has posted, and written, about the fact he rinses his cigars with water before smoking, supposedly to help regulate and even out the burn. I have not tried this or know anyone who actually has. I personally believe Nee was drunk when he posted such a statement. There are people out there who hold every word from this guy as gospel. This is where the idea of rinsing cigars comes from. Not from some secret knowledge that long time smokers posses and found to be true.