Code Snippets are just copy and paste pieces of code. It´s not rocket science to understand. The thing is, there is some code that you will use a thousand times and you can save this code in a special panel in Dreamweaver, the Snippets Panel, and easily access the code(s) and place the code with a couple of clicks.
There is some code that you will only use about 2 times a year. You will forget how it is written. This is another good reason for using Dreamweaver Code Snippets.
Also, if you are doing web design for clients, on some occasion, you will run into situations where you are using code unique to that client in big projects. You can save and use code snippets in special situations like this also.