A conditional simile introduces a restriction under which the comparison would fully hold. The condition is part of the image’s rhetorical logic, not an incidental grammatical ornament.
«عَزَمَاتُهُ مِثْلُ النُّجُومِ ثَوَاقِبًا لَوْ لَمْ يَكُنْ لِلثَّاقِبَاتِ أُفُولُ»
His resolve is like brilliant stars, if only those brilliant stars did not set.
The shared brilliance is qualified by the difference in permanence: the subject is praised as enduring, whereas stars disappear from view. The condition sharpens the comparison.