Deadwooding and crown cleaning are needed when the crown is not big enough to reduce but needs thinning to allow more light to pass through. Often in Larger gardens where there is space for a larger tree to grow bigger, a crown thin is chosen rather than a crown reduction.
With medium and large trees in especially varieties such as ash, a periodic deadwooding is needed and especially in public places such as schools or gardens where children play. Leaving the tree to accumulate too many dead branches could result in injury or damage to property. A deadwooding can often be combined with a general thin.