There are many benefits of registering your domain for more than the minimum one or two years.

Save Money
Registering your domain for longer periods often saves you money through discounts. You also lock your domain in at Today’s prices protecting you from future price increases.
Save Time
Cuts down on the amount of invoices you need to print off for your Accountant.
Stops someone trying to register your domain when it comes up for renewal each year.
Better Rankings
Google considers domain that have been registered for a long period to be of greater value and ranks them higher.