وَإِمَّا لِتَعْجِيلِ الْمَسَرَّةِ أَوِ الْمَسَاءَةِ
Or to bring delight or displeasure to mind sooner.
A speaker may want the first words to give the listener the emotional quality of the message, before the predicate is complete. Tafāʾul means a favorable expectation and taṭayyur an ominous association in this rhetorical discussion.
The main point is timing: by fronting the salient expression, joy or alarm is communicated immediately, rather than only at the end of the whole utterance.