Why do you remember spells in DND?

The setting of casting in DND is different from the blue bar in other games. A mage needs to recite the spells he knows every day before he can release the recited spells on a new day. When the spell is released, it will be forgotten. Moreover, the mage has spells every day, and the number of spells released at each level is fixed. Therefore, the mage's spell position is more cherished. For a mage who is good at playing DND, it is very important to choose which spells to remember every morning to deal with various situations in the day.