My drainage.... best thing ever. it's even automated to drain on it's own and this is great because i don't have to be home or have someone come over when I am away out of town. It also is a safety because If the water were to run on and on for whatever reason it would just pump it away.
Well, since we go through a lot of water with our chameleons (by misting, dripping, etc.) then we face the problem of having that water either pool at the bottom of the cage or leak out and ruin our table-tops or whatnot. So it's easier to make something that your cage can sit on so that the water drains into a bucket or something controlled like that. It just makes life easier! There really aren't any cons, especially if you make it to suit your tastes and your needs.
What you make it out of and how it works it up to the individual and what they want. If you search "drainage" I'm sure you'll get examples from members. It can be as easy as sitting your cage on something like a wire shelf with a plastic tote under the cage to catch the water or as complex as you want it to be.