Why is the starter rotated when a tube light is not working properly?

The starter in the tube light has a function in the beginning only and when the tube light starts working, we can remove the starter from its socket.

What happens is that in the starter there is a valve which is connected in series to the elements of the tube light and the valve shorts for a while and elements get heated up and the gas inside tube light is ionised and starts radiating light and the current circuit completes through the tube light and the starter becomes redundant and its purpose is over.

Sometimes when starter is loose or not functioning properly the rotation of the starter makes the momentary connection with the element of the tube light and the tube light glows.

You might have seen in marriage functions or big pandals the electrician do not use the starter and instead just short two naked wires coming from the tube light to start the tube light by the momentary current through the elements. Then, they remove these two wires from each other and tape them.

I am not a science student to give you a technical answer, but I feel that the starter would give the required push of electric start to make the tube light function normally.

Actually, fluorescent tubes are filled with mercury vapours. They use an electric charge to excite mercury atoms in order to produce light rays. A starter is used in tube light to provide initial current to the filament of the tube light. Therefore you must have noticed that in the starting the starter has its role and once tube light has started it doesn't matter if the started is there or not. So the name suggests a starter is used to start the tube light only.

The role of a starter is in its initial stages only and these days we are getting some electronic chokes where in your tube light don't require any starter. The starter is like a valve. It will be connected to the other elements of the tube light in series. the starter will initially give the required energy to the elements and they will get heated up WIth this heat the gas inside the tube will get ionised. Then starts radiating light.
when we observe that the started is lose we will try to rotate it so that at a particular point it may get connected to the other elements for a moment and the light starts working.

The function of starter is to excite the Mercury atoms inside the tube and as such the ions are ionised in the gaseous medium and finally the tube starts glowing with the attainment of threshold charge. The function of the starter is of short duration and once the tube started its glowing operations, its function stops.
Sometimes the starter may not work initially because of loose contact and then by rotating same, at some points it may release the threshold charge to ignite the atoms of mercury lying in the gaseous medium in the tube.
Yes without the provision of the starter, we can make the tube glow by connecting the wires of the opposite ends so that the current flows inside the tube, once the glowing starts, it will emit light uninterruptedly with the flow of the continious current.