What is the purpose flash that is movable on nikon d3100?

It is kinda awkward to see that…

It retracts so that the tube can be protected when not in use. At least, I think that's what you're asking, your English leaves something to be desired.

The flash needs to be in a higher position or as far away from the lens to prevent casting a shadow from the lens body.

If you want to get into flash photography you would be wise to get a dedicated flash unti for the hotshoe.
The pop up flash is very good for close subjects. Remember when using wide angle that the lens hood, or even the lens may obstruct some of the flash, causing a shadow.
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