In more detail
Because the biggest unit comes first, ISO dates sort correctly as plain text. A full timestamp is date, T, time, then a zone: Z for UTC or an offset such as +02:00. The standard also defines week dates (2026-W40-6, which is Saturday of week 40), durations (P1Y2M3D, PT1H30M) and intervals. Weeks start on Monday, and week 1 is the week containing the year's first Thursday. See the Date & Time Formats Cheat Sheet.
Why it matters
03/04/2026 means 3 April in most of the world and 4 March in the United States; 2026-04-03 has only one reading. A timestamp without an offset is ambiguous about the moment it describes. The week-based year also trips up programs: some date-formatting libraries use one pattern letter for the calendar year and another for the week year, and confusing them prints the wrong year around New Year.
Example
2026-10-03T14:30:05Z and 2026-10-03T16:30:05+02:00 are the same instant. 1 January 2027 is a Friday, so it belongs to the last week of the previous week-year:
>>> datetime.date(2027, 1, 1).isocalendar()
datetime.IsoCalendarDate(year=2026, week=53, weekday=5)