I think I'm on pretty thin ice as far as submission guidelines go, and will take no offense if a mod needs to nuke this post from orbit.

Is there any way you can narrow your question down a bit? You're probably going to get a different answer with each individual rank. Here are a couple:

During WW1, the Brits decided that commissioned officers should be the guys who flew aircraft. Makes sense, right? An airplane is expensive, and a great deal of individual aggressiveness and independence of action is required. Those traits were valued/encouraged in officers, while obedience and disciplined was more valued in enlisted personnel.

Since operating an aircraft is a unique body of knowledge, a "billet" of Flying Officer was created. A billet is not a rank, it's a job description. It's a way for an organization to say,"This dude does Job X to the exclusion of other jobs". One stereotypical example of a billet would be, say, a Mess Sergeant. The Mess Sergeant might be a relatively senior NCO, but instead of leading a section of rifleman (or whatever the other sergeants in his unit do), he is responsible for feeding his unit. Scrounging up chow, insuring everyone eats, cooking, etc. is so time consuming that it's not reasonable to expect that sergeant to perform other duties, hence he's "billeted" as the Mess Sergeant.

For a time early in WW1, you could find Lieutenants (ahem...Lefttenants), Captains, Majors, etc filling the billets of Flying Officers.

As the RAF came into it's own as an organisation, it adopted the term Flying Officer as it's most junior rank. This serves to differentiate RAF personnel from us regular boring ground pounders, and also lets you know that you're dealing with a guy who is strictly an aviator, i.e. not a cavalryman who also knows how to fly.

That's sort of a smooth evolution of a new rank being created. Here's an entirely artificial creation, although one that took place in the US military:

Prior to WW2, there were six enlisted ranks in the US Army and USMC. As the Joint Chiefs looked at the evolution of warfare during WW2 and Korea, they realized that a stronger NCO corps was needed. Warfare had become more technical, and there was a need to decentralize/distribute authority. The JCS needed competent enlisted folks to stay in the military longer, and for more guys to make it a career.

One way they incentivised this was to increase the number of ranks to nine (ten if you count the most senior enlisted position in the individual service like the Sergeant Major of the Army, a position held by one person at a time). This allowed for more frequent promotion. No longer would a competent Marine have to grind out 4 years as a PFC before getting a pay raise and increased responsibility. Now, he could be promoted to the new rank of Lance Corporal after a couple years of service. This allowed more promotion (and it feels great to get promoted, by the way - lots of guys get "hooked" on it) without creating a top-heavy organization.

The US Army took this opportunity to create the rank of Specialist. A Specialist is usually in a technical position - radio technician, mechanic, something like that. He might not be involved primarily in troop leadership (that's what corporals and sergeants are for) but he's still not some wet-behind-the-ears PFC, either. A Spec-4 was roughly equivalent to a corporal in terms of pay, and a Spec-5 a buck sergeant.

I hope that helps, or at least sparks someone else's ideas.