Why we love the snorkel !
The snorkel does a few things:

removes the need to breathe which means it smooths things out and removes any ‘gaps’ in the stroke

allows us to work on body position and alignment… ie. eyes down, spine long, hips high

allows us to work on hand entry being in front of shoulder and not crossing the midline – both above the water and below

all of the above often means we can increase stroke rating which further removes gaps in the stroke and is good for efficiency as well as transfer to OW swimming…