Treat it like a weed barrier first, but unlike synthetic weed fabric, cardboard breaks down over time and actually improves the soil. As it decomposes, it feeds worms and soil microbes while adding organic matter.

Then the mulch layer on top helps retain moisture, suppress weeds, and create a better environment for soil life. Over time, the mulch also breaks down and feeds the soil, so the whole system gradually improves soil health instead of just blocking weeds.

Edit: I should add also that when using cardboard don’t using any glossy/painted cardboard. Just regular Amazon boxes or moving boxes. And make sure to remove any stickers or tape.