A patient once asked her dentist: “which teeth should I floss?” The dentist chuckled and replied: “only the ones you wanna keep”. This is about as rough and raw as the issue gets; if you do not floss or use an interdental toothbrush, you are not getting rid of a good chunk of the bacteria that live in your mouth, and you are risking periodontitis, cavities, and tooth loss. If you want to keep a tooth, you need to remove all plaque that is around it, otherwise the bacteria will start to mess up the tooth or the gums and supporting structure around it.
What flossing does
Dental floss removes plaque from places where the toothbrush cannot get. Plaque is the calcified form of biofilm, the medium in which bacteria, live, thrive, grow and reproduce. What flossing does is destroy and remove these habitats. The main problem is that this biofilm sticks to the tooth surfaces and the acidic byproducts and excretions of the bacteria come in contact with the enamel, causing cavitation,a dn this is why plaque needs to be removed.
There are some minor concerns to mention, though. When flossing, you run the risk of removing the bacteria from one place and simply transplanting it into another space in your mouth. This is why you should start flossing from around your gum downwards, this way, the sensitive area between your tooth and your gum gts the bacteria removed from it, instead of getting a bunch of plaque jammed into it, as would be the case when you go the other way around.
You do not need to necessarily floss, but you do need to clean between your teeth. You cannot get everything removed with just mouthwash, as bacterial biofilm tends to stick to tooth surfaces, so something needs to be used to remove it. Usually, the best alternative is an interdental toothbrush, which has very tiny bristles that poke out in all directions, and can be used to remove the plaque from between the teeth.