If you open the fluorescent casing, 'L' wire goes into directly into magnetic ballast (from your main source/ DB) and out of it then to the pin socket under normal fluorescent wiring. Removing the starter is only breaking up the other loop from left side pin to right side.

Function of ballast is limiting the current flow. The current flow after magnetic ballast is more than sufficient for LED. LED could be wired directly to 'L' and 'N'.
Think of magnetic ballast in this case as a 'middleman / agent'.  Instead of getting your goods from whole seller, one choose to still continue the tradition to get from agent (which more expensive -lah). With or without magnetic ballast the LED still works but consume more electricity.

I am sharing the knowledge as the info is scattered everywhere in the forum and the lighting shop is not much help either. The purpose of this thread is to consolidate the info and adding my experience in changing the normal fluorescent to LED tubes

For better understanding on the fluorescent light wiring (the foundation), this link with pic is quite good   
http://www.louyeh.com/how-to-wire-up-a-sim...orescent-light/

ahem..  sifu here.. (actually its master sifu since i have technical certificate..)

In a standard fluorescent fitting..

The ballast acts as inductor to limit the current and increases voltage (by the fluctuating magnetic field in its coil) so as to make the tube fluoresce via the starter (bimetallic contact)..

hmm..

anyway because it is an alternating current (sinusoidal voltage) there is a power factor where some heating happens within the inductor (ballast) which is the reason for the reactive power loss..

Hell .. I can't explain technical things in a simple way but if you run an ac current through a coil there will be some heating ..

This heat translates as a loss in efficiency of the load..

Edit * Of course in an LED where the current is in the mA it will be negligible thus you can leave it in circuit and it won't matter..

technically there will be some power loss but not in the magnitude of 6-8 Watts (milliwatts in direct proportional to the current) ..