A crown thin lessens the canopy density by around 15% through selective removal of small branches.
This allows increased air flow within the crown, discouraging disease and lessening the sail effect in high winds, which greatly decreases the likelihood of storm damage (root upheaval, snapped limbs etc.).
Thinning also reduces mechanical stress on large limbs which suffer from snow build-up or physical defects such as cracks or cavities.
We normally give a tree a light thin while carrying out other pruning operations within the canopy, especially while performing a crown reduction.