In larger orgs you may have tens or hundreds of recordtypes. For instance, when a user creates a new Opportunity, the first step in the process is to select the correct recordtype from a list of x choices. However, what if, for instance, you have a low-level user that only uses a single recordtype? They don't want to go through that step of selecting the same recordtype each time.
To make this user's life easier, you can make a custom button that bypasses this recordtype selection screen and takes them directly to the new Opportunity page for the correct recordtype. Simply create a new custom button with the following code and place it on the page layout for the user's profile instead of the standard new Opportunity button.