I have ducks b/c they are cute, funny, really entertaining, pretty, nice, small (I have call ducks so they are nice and small), they give me eggs, and the hens go broody and give me lots of fuzzy lil ducklings, which I go and sell.
Entertainment, for SURE! They are fun to watch, they make good conversationalists, the sounds they make in the morning are wonderful, they keep you happy because they make you laugh quite a few times a day! Plus....they make a great conversation starter.
The best reason to keep ducks is Duck Eggs and their unique health benefits and attributes.
They've got slightly different grazing habits and are better garden foragers as they generally leave your veggies themselves alone and focus on weeds and bugs. They also (as mentioned) eradicate slugs and snails and thus are the natural eliminator of liver fluke for sheep raisers. They're also known to reduce mosquitoes and frogs as mosquito larva and tadpoles are like fish in a barrel to hungry duck-zillas.
Serenity, therapy! My duck chores have become a well welcomed part of my day now. I can't wait to get home, throw on my old jeans, and flip flops so I can go out and see my duck babies! And doggies too!
Plus there is a certain entrtainment value to watching them be ducks and do thier duck things.