After passing your IR you will have a sound understanding of the principles of flight, the bread and butter of how airplanes fly.
After gaining your MEIR you'll know about this extremely cool but different way of flying around in clouds and traffic and staying in control with the aid of a ATC.
For me personally, I found I gained the most knowledge with the Instructors Rating.
It really teaches you the basics of flight, the basics every pilot should know.
As far as progression into a job, if your flight school will have you after you gain an IR then that's a way to get started.
From what I've seen of GA operators so far, they're more interested in guy with MEIR because they know that pilot can handle the A/C if they get in the bad wx.
There was one operator that said they don't like flight instructors because they can point out all the defects and rules rather then getting the job done
