All those who had been born in the past forty years had not been at Har Sinai. As a result, Moshe Rabbeinu needed them to hear and understand the עשרת הדברות and understand that all of them whether they had been born before or after the 40 years in the desert (Rabbeinu Bachya). They also had to understand and be attentive to the details, so that they could perform them properly (Rabbi D. Z. Hoffman)
This emphasizes that all of us (including every generation in the future) were as much a part of the covenant as those who had been living at the Revelation at Sinai.
Rav Hirsch say that this begins the third part of Moshe introducing the repetition and explanation of the Torah.
This interweaves the general fundamental feelings of the fear of
Hashem and love of Hashem into those laws, the fulfilling of which was
to accompany the people everywhere in the greater isolation which now
awaited them, as the constant incitement to, and promoter of these
But this deeper going into the laws begins with once again looking
back at the historic event of the revelation on Sinai. Above in Ch.
IV,10 et seq. and 33 et seq. that experience was again brought to
their minds to teach them the full and put purport of our
consciousness of Hashem. Here it was repeated as the foundation of the
task of our lives, and for that, too, the Ten Commandments were also
repeated as the fundamental principles of the whole Lawgiving.