Why it's preferred to use vibrators when pouring concrete in some cases?

to fill the small voids between forms and concrete and to remove excessive air particles.

To eliminate the air trapped in the concrete while pouring and to achieve the max possible density of concrete when harden.

To consolidate and compact freshly placed concrete. For good bonding between the first layer and the next layer of fresh concrete. To eliminate air bubbles on the freshly poured concrete.

The vibrators penetrates concrete equally among on all sides enaboe to reduce labor work.

It's very well mixes and it's solidification prevented.

To get better compaction of concrete it is necessary to use vibrators during laying process of concrete. It helps in placing concrete between reinforcements, voids and remove excess air voids. By doing so we can ensure the maximum strength of the concrete. Excess use of vibrators should be avoided as it segregates aggregates and cement slurry and develops honeycombing in the concrete work.

Air can be entrapped while pouring concrete, by using vibrator entrapped air is eliminated, consolidating well, producing rich denser concrete, and getting the desired strength of concrete.

In order to remove air voids from the concrete. So that in future while de-shuttering honeycombing cant take place.

there  is air in the concrete by using vibrator we eliminate that air from concrete for good strength

To condensr the concert. So that bubles air missing and the concert bocomes solid.

The vibrations cause the mixture to liquefy, reducing the internal friction of the mix components of cement, aggregate, and water. This allows the mix to move around more easily in whatever form you are placing the concrete. ... The vibrations help to bring those air bubbles to the surface etc