Imperatives can move beyond a straightforward order. Permission (الإباحة) removes a restriction; challenge exposing inability (التعجيز) demands what an opponent cannot accomplish; threat (التهديد) appears to leave a choice while warning of its consequence.
«اعْمَلُوا مَا شِئْتُمْ»
Do whatever you wish.
In its Qur'anic context this is a threat, not unlimited permission.
«فَأْتُوا بِسُورَةٍ مِنْ مِثْلِهِ»
Then produce a chapter like it.
This challenges the addressee and exposes inability; it is not a neutral request for creative writing. The intended function depends on the addressee's ability, circumstances and the speaker's purpose.