After concrete is placed within its forms, it is frequently worked with an industrial vibrator which can be inserted into the newly placed concrete while it is is at its most plastic stage(soon after placement).
A mechanic who is very skilled at this operation is required because the amount of vibration must be tightly controlled to produce the desired results without separating the components of the Concrete. Vibration gets the fines of the mix into all the spaces within the form. Corners of concrete forms have a tendency not to get filled, leaving voids (volumes where concrete should be, but is not, like inside corners of the form and complex shapes within it).
Vibrating the concrete helps in placement within these problem locations, and produces a uniform surface to the edges of the kind right next to forms.
Without a vibrator, there would be gaps in placement and these would result in unsightly surfaces in the finished material.