What was one reason Muslim scholars developed an accurate calendar?

Every civilization needs some sort of calendar.

Explanation:

A farming society would need it for sowing at the right time, for one thing.

Later on, the need for accuracy was felt, to mark certain days as special, and especially as the muslims spread over great parts of the world (as did the Christians before them), it was a means to cohesian within the Islamic world, just as the Christians had their special days at the same date all over the Christian world.

(as had the Mayas, Incas, Buddhists, Hindi, and the rest).

To keep track of religious holidays.