Why do the letters איו of the word אליו have dots over them?
(18,9) “And they said to him (אליו): Where is Sarah your wife?”
The gemora in Bava Metzia 87a asks why the letters איו of the word אליו have dots over them. It answers that the Torah is teaching us that a person should enquire of his host - asking the man about his wife and the wife about her husband.
The Midrash elaborates on this and brings the teaching of R. Shimon ben Elazar, that whenever the undotted letters outnumber the dotted letters, you expound the undotted letters, and when the dotted letters outnumber the undotted letters, you expound the dotted letters. Here the dotted letters outnumber the undotted letters, so we expound the dotted letters - איו - “Where is he?”. From this we learn that just as the angels asked Avrohom “Where is Sarah?”, so too they asked Sarah “Where is Avrohom?”.
But the Nimukei Yosef asked why it was necessary to add dots to the letters איו in order to force us to expound those letters - the Torah could have simply put a single dot over the letter ל and we still would have expounded the letters איו in accordance with the teaching of R. Shimon ben Elazar!
Many answers have been given to this question but it seems to me that the letters which are dotted are uprooted, as Rashi explains in the gemora quoted above, as if to say that their real place is not here but somewhere else, as I will explain:
The gemora in Eruvin 53b writes that R. Yossi HaGalili asked Beruria the wife of R. Meir “Which is the way to Lod”, and she berated him by pointing out that Chazal teach “do not converse excessively with women” (Pirkei Avos 1:5), and thus he should have spoken more briefly and said "Which to Lod?". From this we learn that one who speaks with a woman has to speak as briefly as possible.
Thus we can explain that the reason why the letters איו are dotted is to teach us that although the angels asked about Avrohom, they did not say explicitly “Where is Avrohom” (איה אברהם), but rather they said “Where is he?” (איו), in order to minimize their speaking with Sarah according to halachah.
But really this rule only applies to humans who are at risk of sinning because of their evil inclination, but angels who do not have an evil inclination do not have to be concerned about this. Therefore the angels did not need to speak briefly and could have have said explicitly “Where is Avrohom”. Therefore the main teaching of the dots is for us, to teach us they we should speak briefly with a woman when enquiring about her husband.
This is why the dots were put specifically on the letters איו and not on the letter ל, to teach us that the message of saying איו - minimizing speech with a woman - is not for here but for elsewhere. This answers the question of the Nemukei Yosef very well and we see that the language of the Midrash is very precise.