etiquette: noun | et·i·quette
the customary code of polite behaviour in society or among members of a particular profession or group
synonyms: protocol, manners, accepted behaviour, rules of conduct, decorum, good form
Etiquette is important for several reasons, one of them being that if you don’t abide by it, no one else will enjoy being around you.
With code review, not abiding to etiquette means that people will not enjoy reviewing your code, sending code your way for reviewal, or both.