Al-nakīthah and al-jullā both concern adversity, but not with identical emphasis. The former evokes a difficult entanglement calling for sustained effort; the latter a grave, exceptional crisis.
ExpressionSense
أَمْرٌ لِلنَّكِيثَةِA hard affair requiring exertion
أُدْعَ لِلْجُلَّىBeing summoned to a great crisis
مِنْ حُمَاتِهَاJoining those who defend in that crisis
Together the terms make the speaker’s claim of steadfastness more demanding and forceful.