Question

why is important to brush one's teeth before going to bed

Answer 1

The bacteria produce acid throughout the day, and even more when you're eating, which is why you want to get rid of the film at least twice per day—any less and the acid byproducts may eat into your enamel and cause cavities.

Answer 2

HEY DEAR.....
HERE'S YOUR ANSWER⬇
It is important to brush and floss your teeth twice a day to get rid of the acid producing, harmful bacteria inside your mouth. After you have your food, you'll have lots of bacteria left in your mouth and if you don't get rid of it, then they will secret acid which affects the pH level of your mouth and results in cavities.
Any doubts, please ask me in the comments.
I HOPE YOU FIND IT HELPFUL.
@ LB :)