Not necessarily the part for calculating the day of the week for any arbitrary day centuries ago, that’s just a useless party trick, but for the current year so you don’t need to pull out your phone to check. Knowing that 1/3 (or 1/4 on a leap year), the last day of February, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12 are all the same day of the week, that this year they’re all Tuesdays, and next year they’re all Thursdays, is mostly easy to remember and very frequently useful.
The Doomsday rule.
Not necessarily the part for calculating the day of the week for any arbitrary day centuries ago, that’s just a useless party trick, but for the current year so you don’t need to pull out your phone to check. Knowing that 1/3 (or 1/4 on a leap year), the last day of February, 3/14, 4/4, 5/9, 6/6, 7/11, 8/8, 9/5, 10/10, 11/7, and 12/12 are all the same day of the week, that this year they’re all Tuesdays, and next year they’re all Thursdays, is mostly easy to remember and very frequently useful.