Dock Clock 3 is a simple app that shows you the time from the dock. The default analog clock icon is very customizable. You can change the colors and sizes of the hands, and the changes that you make are instantaneously reflected on the icon. There are eight clock designs that use different backgrounds. These look better than the default icon, but you can't customize them in any way. Some of the clocks have numbers on them, but some don't.
When you first run the application, you will see a welcome screen that explains what the application and how you can customize it. Basically, the app runs on the Dock, and you can right-click on the icon to access the Preferences window (where you customize the clock) or the about window. You can also left-click on the clock and use the menubar to access the aforementioned windows.
In short, Dock Clock 3 is a nice app, it works as advertised and it gives you a bit of freedom to modify the default icon as you want. Personally, the menubar, digital clock is all I need to check the hour. I don't know why someone would need a clock on the dock, but if the need arises, Dock Clock 3 can satisfy it.