The meaning becomes clearer when the condition and its response are identified together.
ConditionResponse or consequence
مَنْ صَلَّى عَلَيَّ وَاحِدَةًصَلَّى اللَّهُ عَلَيْهِ عَشْرًا
مَنْ بَنَى لِلَّهِ مَسْجِدًابَنَى اللَّهُ لَهُ بَيْتًا فِي الْجَنَّةِ
مَنْ رَأَى عَوْرَةً فَسَتَرَهَاكَانَ كَمَنْ أَحْيَا مَوْءُودَةً
مَنْ سُئِلَ عَنْ عِلْمٍ ... ثُمَّ كَتَمَهُأُلْجِمَ يَوْمَ الْقِيَامَةِ
The letter فَ introduces the response in some constructions, but is not obligatory in every example. The key idea is that an action or condition is followed by a stated moral outcome.