Seaweed puts up with a lot - it's constantly thrashed about by waves, tides and currents - so it has evolved to be rubbery and flexible. Alginate, an alginic acid compound, is key to this suppleness.
Amazingly, when seaweed breaks down in soils, this alginic acid attaches to naturally existing metal molecules. This strengthens the soil structure, providing aeration and water-holding capacity. As seaweed breaks down it also increases the colony count and metabolic activity of microbes in the soil and provides multiple macro and micro nutrients.
Top tip:
To avoid a strong decomposing seaweed smell, cover it with another mulch or simply add the seaweed to a compost bin.
References: https://www.frontiersin.org/articles/10.3389/fpls.2019.00655/full https://link.springer.com/article/10.1007/s10811-015-0544-2