Sublight Speed, Worth anything?

Since ships in tactical combat seem only to fly straight at one another as fast as they can (vs. using speed to try to keep the range open to exploit longer ranged weapons vs enemy for example). What is the benefit of being faster?

In essence, why buy Thursters since you are going to close with the enemy no matter what?  I must be missing how they help. Anyone able to shine a flashlight onto this for me?

-Mettius