I originally put this together because I needed a bottle with a narrow spout which would make it easier to get the water into the narrow gap at the end of seedling trays. Since I'm starting them early I have to keep them inside which for me is in front of my big bay window in my nice carpeted living room where there's plenty of light. I like to water them from the bottom which means pouring about a pint of water through a half-inch gap at the end of the tray and hoping I don't spill any.
This introduces two issues which I solved using this decanter: accuracy and quantity. Since the bottle is clear and already close to the right quantity for one tray it makes it easy to measure the amount of water I'm adding to each tray. Granted this creates extra work since I'm constantly shuffling back and forth to the kitchen for more water. Perhaps this calls for a large watering container: maybe something along the lines of a two-liter Coke bottle.
Secondly, this is much easier than just pouring the water on my plants at work. I have more than a dozen plants at my desk, some of which dry out before others. Most of the plants are large pots that only need water once a week and the whole batch can be taken care of in two or three trips to the restroom sink.
I also have several small cacti which dry out completely on an almost daily basis. Instead of running them to the restroom sink every day it's nice to have a watering bottle at my desk. Plus they are crowded together on a tray and the long spigot makes it easy to get to all of them without spilling any water. Finally, and most importantly, the water flows slowly which keeps me from overflowing the pots and making a big mess.
The real reason I made this is that, in true Instructables spirit, I had some spare time, love putting things together, was thrilled at how well all the parts integrated, and hoped somebody else might get some use out of the thing. Remember, this is my first instructable. Granted it's not a Traditional Polynesian Ice Canoe - I wish I had some ice nearby so i could try that - but it was fun to do.
Now...without further adieu...