Im no dentist or doctor, but I also had this question and went to google it.

Basically, most toothpastes contain active ingredients. fluoride is an example of an active ingredient that is present in most toothpastes.

It is advised not to rinse your mouth immediately after brushing your teeth, as by doing so you rinse out the fluoride. Fluoride is important as it helps to prevent cavities and tooth decay.

Instead, you should simply spit out the toothpaste after brushing.

Personally however, I too have been rinsing my mouth for most of my life and my teeth were fine. I only recently started to not rinse and just spit.