Complex sentence: The principal purpose, however, of the "high and tight" is to prevent lice, promote general hygiene, and with modern regulations against beards to ensure a good seal is made around the face when using a gas mask.
Split into simpler sentences: The principal purpose, however, of the "high and tight" is to prevent lice and promote general hygiene. Modern regulations against beards also ensure a good seal is made around the face when using a gas mask.