I personally use it to plan out designs and store them for later use or to get a rough example of how something might look in game before I spend the time and resources making it (I could just use creative (which I do) but for some things I find Tiled easier and quicker to use). I also find it to be a good method of sending plans to someone, they are usually easy to follow and you can export what you make as an image.
Here's an example of some cobblestone generator designs I've started marking out from different sources. These ones don't use Redstone.
I am sure you can use to design or make anything really though.
It's really simple to use, just start a new file, then go up to Map > New Tileset and select your terrain.png or items.png file from either inside your texture pack or inside the Minecraft jar. Then change the dimensions to match the size of your texture pack (i.e. 32x 64x 128x etc..) and that's it. It supports layers and I think even scripts/commands of some sorts but I haven't gone into learning them yet.
You can get Tiled (it's free): http://www.mapeditor.org/
I hope you find this tool as useful as I do. Share some things you make with it if you use it.