What is the use of battery in diesel generator?

The battery used is to turn over the engine. Once the engine is running, there is an alternator that is charging the battery back up. To remove the battery once it is running is a no no. The voltage out put of the alternator in the engine's own charging system can do harm to electronic systems as the voltage can go sky high(120 volts plus) and unregulated. This ,you DO NOT want to happen. This has nothing to do with the main Generator driven by engine

Some generators rely on the battery to power monitoring circuits: the idea is to provide a constant voltage source, where the driven-load may fluctuate with load. If you disconnect the battery lead, a good example may be the shut-off valve for the diesel fuel line may close, turning off the engine.

In a diesel Generator, the Diesel part only refers to the Generator driver engine. It doesn't do any of the actual generating of the electricity. It's the same as any other driver, steam or gas turbine or gas engine...etc. The battery of the diesel engine is purely for start-up purposes. Once running, the battery can be disconnected but, it won't be re-charged and, it will have to be re-connected to restart the system after a shut-down.

Nothing, in all likelihood. The battery is there to start the engine, to warm the glo-plugs (if installed) and to turn over the engine until compression starts it. It's not needed once it's up and running, although it's possible electric interlocks might not allow it to run. But technically, it can be run without any battery.

to start the motor and to provide current to the field coils in the alternator to start the alternator producing electricity. once the alternator is making electricity, the battery can be disconnected because part of the alternator output goes back into the field coils so it feeds itself.

A Diesel Generator fires by itself because of high compression ratio. The battery is used for the starter motor. If you remove the battery while the engine is running, the engine continues to run.

If it has a working alternator, nothing. You will have to put it back to restart it of course if it stalls. Also if it stalls it either doesn't have an alternator or the alternator is bad.

nothing it doesnt need the battery to run unless its a newer diesel that needs power for electronic injectors

The battery is to run the starter motor. Once the diesel engine is started it will run until you stop the fuel flow or block off the air intake.