What you're referring to is called "Voice Procedure" (or sometimes "Vocal Procedure"). It was formulated not long after the invention of the telegraph, and is a way of carefully enunciating specific letters/numbers in order to minimize confusion on the other end. With the advent of widespread radio communication in the First World War, this standard was modified and adopted by the major militaries of the world.
As most sci-fi militaries take their cue from modern militaries, this form of spoken communication has been used for what we see on-screen as well. For instance, it can be seen in Star Trek: The Next Generation whenever coordinates are called out:
PICARD: Mr. La Forge, take us out of orbit.
GEORDI: Destination, sir?
PICARD: I don't care. Let's just get some distance between us and this system.
GEORDI: Aye, sir. Course 9-7-0 Mark 3-1-8, speed... warp 3.
RIKER: Where will that take us, Mr. La Forge?
GEORDI: The Opperline system.
RIKER: An interesting choice. Why?
GEORDI: Curiosity. We've never been there.
Use of the term "decimal" is proper according to the standard, but the word "mark" is sometimes used to indicate a decimal or exact point in time, depending on the context of the message at the time. Using "decimal" instead may have been an attempt by Lucas to make the Empire seem more alien to a modern audience.