Why it's prefered to use the vibrator when you pour concrete in some cases?

Vibrators are used to settle down the stone materials in concrete an also to prevent air gaps in between concrete.Air gaps in concrete can create weak points.

Vibrator are used to fill up the gaps and pores where the concrete is poured. It is being done because the concrete material are often very bulky to settle down on the surface by its own self. Therefore vibrators are used.

Just to enhance its workability and clearing air pocket to strengthen the concrete. It depend upon the structure also. But timing for vibrator use and the distance must be followed as per specification.

In pouring concrete air  pockets create a weakness in your concrete so vibrating is a means of ridding one self of the air pockets hence vibrating fo small jobs it is not required on larger jobs well it must be vibrated