On Sinai, Moses intercedes for Israel, asking Yahweh to go with them. First, Yahweh promises to send His Angel ahead; finally, He promises to go before Israel Himself.
The sequence of events from Exodus 19-40 is a double-covenant sequence. Israel agrees to do all that Yahweh commands, and Israel’s representatives eat and drink in Yahweh’s presence to seal the covenant (Exodus 23-24). Then Israel breaks covenant (Exodus 32), and Moses intercedes for a new covenant (Exodus 33-34), which Yahweh grands (34:10).
This small double-covenant sequence foreshadows the large covenant sequence of Scripture: First Yahweh sends His Angel, and then Yahweh comes in person. First the Angel, then Jesus. Moses, we might say, is pleading on the mountain for incarnation.