Ihtiras inserts wording that prevents an unintended interpretation of the preceding speech.
فَسَقَى دِيَارَكِ غَيْرَ مُفْسِدِهَا صَوْبُ الرَّبِيعِ وَدِيمَةٌ تَهْمِي
May spring showers and continuous gentle rain water your dwellings without damaging them.
A prayer for abundant rain could unintentionally suggest destructive rainfall. «غَيْرَ مُفْسِدِهَا» closes that possibility and preserves the intended blessing.
The addition does not create a different central scene; it protects the intended meaning from a plausible misunderstanding.