What did Moshe mean when he told the people that Hashem came "Lenasos eschem"?
Ramban #1: What he meant was that by appearing to Yisrael directly, and revealing the Shechinah to them, He got them accustomed 3 to Emunah, injecting it into their hearts, to cleave to Him forever.
Ramban #2 (citing Moreh Nevuchim): The revelation was to prepare Yisrael for future tests - inasmuch as, should a false prophet come to negate what they had heard, having seen the Emes with their own eyes, they will be able to overrule his claims.
Ramban #3: It means literally, that Hashem was coming to test them, 4 as to whether they would now keep all His Mitzvos, having driven all doubts out of their minds - to see whether they really loved Him and wanted Him and His Mitzvos. 5
Rashbam: He meant that Hashem appeared to them directly in order to rebuke them.
Seforno: Hashem appeared to Yisrael Panim el Panim 6 with His celestial retinue, in order to accustom them to prophecy
Moshav Zekenim: He tested them with 10 voices. Each was stronger than the previous, and they stood through all of them, even though their Ru'ach departed with each.
A privilege that no other nation has ever merited.
Since a master will test his servant's loyalty in one of two ways; sometimes he will test him by placing upon him a heavy workload - testing him by the way he undertakes to handle it; and sometimes He is particularly nice to him, to see whether he acknowledges what he did for him by working harder and showing him more respect, as was the case here (Refer to 20:17:1:1** [See Ramban]).
What did he mean when added "u'Va'avur Tih'yeh Yir'aso Al Peneichem ... "?
Rashi and Ramban #1: When Yisrael saw Hashem in His awesomeness, they knew that there was none other besides Him and they would fear him (and refrain from sinning).
Ramban #2: The fear of the fire that they were seeing would prevent them from sinning.
Mesilas Yesharim (Perek 24): The primary fear is that a person is always trembling, even when he does not see a pitfall, his heart is worried lest a pitfall is in front of him, and he is not careful of it. Through such fear, one does not come to sin, and if he does, it is like Ones.
Moshav Zekenim citing R. Yehudah ha'Chasid: What was the merit? They said, "if we continue to hear Hashem's voice, we will die" (Devarim 5:22)! The merit was, they did not impose on Hashem to speak softly, rather, they trusted Moshe.
Moshav Zekenim citing R. Yehudah ha'Chasid: There were prophets without this merit (Shem, Ever, the Avos, Bil'am, Iyov)! Through this, they merited famous prophets who prophesied even about other nations, e.g. Yirmeyah, Yeshayahu and Yechezkel. Even though Bil'am prophesied about Kini, Ashur and Mo'av, this was by himself. Hashem sent prophets only from Yisrael.
How would Hashem's revelation result in Yir'as Shamayim?
Rashi and Ramban #1: By witnessing the awesomeness of Hashem, they would realize that there is none like Him, and therefore fear Him.
Ramban #2: Fear of the great fire that they were seeing would prevent them from sinning.
Seforno: Seeing the Shechinah would plant the fear of Hashem in their hearts and prevent them sinning. 1
As R. Yehoshua said to the Emperor of Rome (See Chulin, 60a) 'If you say that you cannot look at the sun, which is merely one of Hashem's creations, how much more so can you not look at the Shechinah itself!' (Seforno).