Verse 23 gave verbs of speed. Verse 24 gives a concrete comparator: an ostrich. The reader can now imagine not only that the camel moves quickly, but how its steps and body appear in the evening landscape.
ʿAshiyyatan is an adverbial expression of time. Bi-qarīb introduces a specific quality within the comparison. The simile gains force by joining timing, movement, and bodily proportion.