If they are consistent and fair the children get to understand quickly what is acceptable and what's not. If you are inconsistent they become confused and may become scared or withdraw as they are unsure of whether their actions are leading them into trouble or not.
if you treat children differently and react to individual children's behaviours differently you can be accused of favouritism, bias and victimisation. you can project an image of someone who lacks control and doesn't know what they are doing and therefore lose authority and respect.
if the school works as a whole to be consistent in line with policies and procedures then everyone - staff, pupils and parents have an understanding of the rules, boundaries and behaviour expectations as well as any sanctions that will result from transgressions. this results in a calmer, smoother day and easier year group to year group transitions.