Tafsir of Al-Hijr 15:77

"Indeed in that..."

(Meaning: In what has been mentioned regarding the city or the villages, or in the fact that they are in plain sight of people who witness them as they pass by them) "is a sign" (a great one) "for the believers" (in Allah the Exalted and His Messenger, may Allah bless him and grant him peace; for they are the ones who know that their evil conduct is what left their dwellings empty).

As for others, they attribute this to coincidence or celestial alignments. Regarding the use of the singular form of "sign" here after the plural forms mentioned previously, it has been said: because what is witnessed here is the remnant of the ruins, not the entire narrative as in what preceded. It has also been said: [the singular is used] to indicate that for the believers, a single sign is sufficient.