A concrete vibrator is used to place the concrete in position and it's also helps in removing the entrapped air particles from Concrete with a motto to increase it's density.
A concrete vibrator is used to ensure that concrete becomes compact during placing i.e. the concrete is placed without inclusion of air bubbles that would affect its strength. This is necessary in most cases because the fluidity of concrete is limited by the quantity of water used in the mix.
The vibrator is used in fresh concrete, during concrete pours, so that the air gaps that might exist in wet concrete are filled in with concrete, therefore, preventing any honeycombs (bubble gaps that remain in concrete) after hardening which will reduce the strength of concrete.
There are many examples where vibrators were not sufficicently used and the hardened concrete demolished and poured again which is very costly.
Therefore a good concrete crew will always use vibrator during concrete pours to shake the concrete to fill air gaps. Important note to remember here is, using the vibrator too much at one single location will cause the concrete to segregate into smaller and larger aggregates which is an unwanted effect.
The ideal concrete must be homogeneous that must contain everything, fine and coarse aggregates together. So do not use a vibrator for more than a few seconds at a certain location.