Linked from
The 15 pages that link to Lunar month, each with the reason it gives.
IntercalationRelated: Its length does not divide evenly into a solar year, motivating leap months.
CalendarRelated: Its length motivates the alternating month patterns of lunisolar calendars.
Babylonian calendarRelated: Babylonian months alternated between 29 and 30 days to track the Moon.
Hindu calendarRelated: Many Hindu calendars organize months around lunar cycles.
Lunar New YearRelated: Lunar months make the calendar date shift against the Gregorian calendar.
Islamic New YearRelated: Twelve lunar months make the Hijri year shorter than a solar year.
Zu ChongzhiRelated: Its measured length determines how lunar months can be fitted to the solar year.