It can be used as a greeting (obviously at night) and also used as a farewell or if someone's going to bed

The thing is that the term "evening" can be translated into Spanish as "tarde" but also as "noche" Because we don´t make the diference between early night and night. Once it is dark it is "noche" and not "tarde" anymore. As for the term "buenas noches" is a Spanish phrase that when translated to English means good night. The term can be also used as a greeting as well as a farewell in its native form unlike the English translation which can only be used in one occasion.

both!