Ighal adds something at the end of an already meaningful expression that heightens or completes a rhetorical effect, although the primary sense was intelligible without it.
وَإِنَّ صَخْرًا لَتَأْتَمُّ الْهُدَاةُ بِهِ كَأَنَّهُ عَلَمٌ فِي رَأْسِهِ نَارُ
Sakhr is so exemplary that even guides follow him, as though he were a lofty landmark crowned with fire.
The first part already establishes his leadership. The final simile turns prominence into a striking visual image. Such a beneficial closing addition characterizes ighal.