It only prevents it when it’s there.

Thr cardboard/newspaper prevents germination of new weeds, and blocks off light from old weeds, generally killing them.

I used this to reclaim a wild patch into a garden bed with cardboard and pine needles as mulch. Took a few months, but it even killed some blackbeery brambles. They came the fuck back of course, but it was enough to get the patch going right.

Dont use colorful or waxed cardboard, and remove all tape. Its annoying, but better than picking it out of the garden later.

It doesn’t, but keep in mind that most weeds grow from seeds or roots in the immediate vicinity of where they end up. Even if you wipe out all of the above-ground weeds, there are typically thousands of dormant seeds and root fragments that will rapidly repopulate the area in response to the soil disturbance and lack of competition after this removal.

Proper sheet mulching prevents these roots and seeds from immediately repopulating the space. You’ll still have weeds invading from border areas and the occasional long-term dispersal but these will be much smaller in numbers and more easily handled by other methods. If the latent weed population in a larger space can be eliminated then the amount of weeding needed can be very low.

Also, very few weed seeds can establish in more typical mulch material like straw or wood chips. Thick layers of organic material require a large energy reserve to push through, usually provided by an existing root system. So once the process is completed, you can prevent recolonization with more ordinary mulching methods, though you’ll still have to fight at the borders if large weed populations exist there.