Sforno on Exodus 19:21/ספורנו על שמות י״ט:כ״א

1רד העד בעם פן יהרסו, when I will be speaking to them. Perhaps they will think that because they have been found worthy to have prophetic insights that they have attained the level of “face to face” prophecy as had Moses their leader. This could prove fatal if they tried to cross the boundary separating Me from them. Your presence down below will act as additional safeguard.