Why, throughout the Torah, does the Pasuk add the word "Leimor"?
Ramban #1 (citing the Redak): When it in connection with Yisrael, it means that Hashem spoke to Moshe to tell Yisrael; here it means to tell Pharaoh. 1
Ramban #2: The Torah adds "Leimor" for clarity - particularly with regard to the prophesies of Moshe Rabeinu, 2 to whom Hashem spoke "mouth to mouth, and not in riddles" 3 (as opposed to all other prophets, whose prophesies were generally ambiguous and had to be deciphered).
See Ramban, who gives numerous examples where it is impossible to learn like that - one of them in Vayeitzei (31:29),where Lavan said to Ya'akov "v'elokei Avichem Emesh Amar elai Leimor"; another in Acharei-Mos (19:1,2) "Vayedaber Hashem El Moshe Leimor. Daber El B'nei Yisrael ... ".
Though it pertains also to other people, who used the word 'Leimor' to make a clear statement (for example Lavan [Refer to 6:10:1:1*], who told Ya'akov that Hashem had spoken to him the previous night and had warned him in no uncertain terms, not to harm him [Ya'akov] and Yisrael [See Korach, Bamidbar 17:27], who vigorously complained to Moshe that all was lost [Ramban]).