Question

give reason why Mecca is the most important city for Muslim

Answer 1

Mecca is the most important place for Muslims as prophet Muhammad was born there or Mecca is the birthplace of prophet Muhammad.

Answer 2

because
The importance of Hajj (Pilgrimage to Mecca) is immense in Islam as its the 5th pillar of Islam. Obligatory for all Muslims who are financially and physically able to visit (at least once in lifetime)
If someone’s Hajj is accepted (by Allah) then he or she will be immaculate because all prior sins will be forgiven.