A principal rhetorical aim of annexation is brevity. A compact genitive phrase can be preferable to a longer relative construction when the intended identification is already clear.
لِأَنَّهَا أَخْصَرُ طَرِيقٍ إِلَى إِحْضَارِهِ فِي ذِهْنِ السَّامِعِ
Because annexation can be the shortest appropriate way of bringing the referent before the hearer's mind.
For example, «صَاحِبُ الدَّارِ» may be more economical than «الَّذِي هُوَ صَاحِبُ الدَّارِ». This is not an assertion that iḍāfah is invariably the fewest possible sounds: the economy must serve the speaker's specific communicative purpose.