1. To talk about God to others esp. children

Deuteronomy 4:9
Only be careful, and watch yourselves closely so that you do not forget the things your eyes have seen or let them slip from your heart as long as you live. Teach them to your children and to their children after them.

2. To keep God number one in our lives

Deuteronomy 4:23
Be careful not to forget the covenant of the LORD your God that he made with you; do not make for yourselves an idol in the form of anything the LORD your God has forbidden.

3. Our salvation

Deuteronomy 6:12-13
12 be careful that you do not forget the LORD, who brought you out of Egypt, out of the land of slavery.
13 Fear the LORD your God, serve him only and take your oaths in his name.

4. To follow His commandments

Deuteronomy 8:11
Be careful that you do not forget the LORD your God, failing to observe his commands, his laws and his decrees that I am giving you this day.