Why was calculating Easter an important reason for Gregorian calendar reform?

Calculating the date of Easter was a primary driver for the Gregorian calendar reform because the Julian calendar had become increasingly inaccurate over time.12

The Council of Nicaea had decreed that Easter should fall on the first Sunday following the first full moon after the vernal equinox, which was fixed at March 21. However, because the Julian year was slightly longer than the actual tropical year, the calendar drifted by approximately one day every 128 years. By the mid-16th century, this drift caused the vernal equinox to fall 10 days earlier than its designated date of March 21.3214

Because Easter is the most important feast in the Christian church and determines the timing of other movable feasts, ecclesiastical authorities required a calendar that accurately tracked the seasons to maintain the integrity of the church year. Pope Gregory XIII implemented the reform in 1582 to realign the calendar with the solar year and restore the vernal equinox to March 21, thereby ensuring a consistent and accurate method for calculating Easter.312