שמות לב ,טו : וּשְנֵׁי לֻחֹת הָעֵׁדֻת בְיָדוֹ
The Sforno explains the reason for bringing down the Tablets. Since the Jews had already sinned and were not worthy of receiving them, why not leave them in the Heavens?
The answer he gives is that if they repent at once he will be able to immediately give them the tablets. And if not, then the shattering of the Tablets will hopefully shock them into repentance.