Line training takes you through all the aspects of managing the operation of the aircraft and time. When to carry excess fuel when not to, ensuring that as far as ATC can permit you are at the best level for your weight the temerature the route you are flying. Keeping all the paperwork running and trying to ensure that all the ground servicesyou need are ready for your arrival. If you arrive somewhere that you don't normally uplift fuel and today you require it if you failed to request it on the descent you may not get it in time to allow an on schedule departure.
You will also fly the aircraft in different weather conditions with the weight and balance in different areas of the envelope to that which the simulator was set at, it's the real life thing rather than constant emergencies and QRH drills.
In short the honing of the skills you have obtained in the simulator, and development of the time and personel managment skills you will require.