Why do calendars need intercalation to stay aligned with the Moon or the seasons?

Calendars require intercalation because the cycles of the Moon and the Earth do not align perfectly with a whole number of days or months.12

• Solar Alignment: A solar year—the time it takes for Earth to orbit the Sun—is approximately 365.25 days. Because this is a fractional number, calendars like the Gregorian calendar must periodically add a leap-year day to remain synchronized with the seasons.21
• Lunar Alignment: A lunar calendar based on 12 synodic months is only about 354 days long. Since there are approximately 12.37 lunar cycles in a solar year, a lunar calendar will drift away from the seasons unless an entire month is intercalated periodically.213

To manage this, many systems use the Metonic cycle, a 19-year period that incorporates seven intercalary months to keep the lunar and solar years in step.435