The reason for this line's inclusion is that it can actually increase the speed of the missile!!

Note: to fully understand the concept of sub-clicks, have a look at Philip's tutorial.

Here's how:

The way AI Wars deals with missiles is that they move 4 spaces when first launched, a further space for every line of code that is processed while they are still active, and a further 4 spaces at the end of each cybug's turn (assuming they have not collided with anything). Therefore, this code:
 
• launch missile
• move forward
• move forward
 
... is not really very efficient. The missile will travel 4 spaces in the click. In the next click, it will travel one space for the move forward command and four spaces at the end of the click. The same for the next click. So in 3 full clicks, the missile will only travel 4 + 5 + 5 = 14 spaces.
 
Consider instead if the cybug's code looked like this:
 
• launch missile
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• assign v1 50
• move forward
Here, the missile travels 4 spaces when first launched. The next click contains 10 commands, so the missile travels 10 spaces, and a further 4 at the end of the click. So in this example, the missile has travelled 4 + 10 + 4 = 18 spaces in just 2 clicks (a significant improvement, which gives the enemy less chance to get out of the way).

Note: the maximum number of sqaures that a missile can travel in one click is 14 (apart from the first one, in which it always travels 4 squares) so if you missiles are not going this far, you might try altering your code to change the situation.

By the way, "beep" is a very useful debugging command. Instead of having to trawl through a debug report, which slows the battle down when it is being created, you can just run a battle and listen to whether your cybug beeps when it should.