"Why, throughout the Torah, does the Pasuk add the word "Leimor"?
Rashi: Refer to 6:12:1:1.
Ramban #1 (citing the Radak): When it is said 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 - e.g. in Parshas Vayeitzei, when Lavan said to Yaakov, "V'Eilokei Avichem Emesh Amar Eilai Leimor" (Bereishis 31:29); another in Acharei-Mos, "Va'Yedaber Hashem El Moshe Leimor. Daber El Bnei Yisrael ... " (Vayikra 19:1-2).
Ramban: Though it pertains also to other people, who used the word 'Leimor' to make a clear statement (for example Lavan, (see the preceding note 6:10:1:1*) who told Yaakov that Hashem had spoken to him the previous night and had warned him in no uncertain terms, not to harm him (Yaakov); and Yisrael, who in the aftermath of Korach, vigorously complained to Moshe that all was lost - Bamidbar 17:27).
