The effect is very obvious. I'm not ashasmed to say that since the age of 18 - your last year of free dental care in Sweden - to the age of 31, I didn't go to the dentist at all.
I brushed mostly regularly the second half of this period, I never flossed. I'm a computer geek, I can get caught up in code for days without brushing my teeth but try to do it regularly.
The result was 13 years of plaque and two visits to a dentist to clean it all out.
Dental plaque is a coating over your tooth and your gums will grow over this coating. So I think it's pretty obvious that dental plaque can cause cavities between your gums and your teeth, infections in your gums and all this will in turn lead to periodontitis.
And also you have saliva glands at the bottom of your mouth that bring with them additional minerals and stuff that get stuck to your teeth, forming plaque. So the front bottom teeth are even more important to floss.
Now I floss regularly and I'm proud to have actually made it one of my daily habits.