Is the foam mentioned in Surat Ar-Ra'd, verse 17, useless as stated in the Tafsir, or does it have benefits for the skin, and would that contradict the noble verse?

The verse in Surat Ar-Ra'd (The Thunder) contains two parables for truth and falsehood. The foam is a parable for falsehood, which has no benefit, because it is a mixture of dirt and quickly vanishes, so it does not last to be beneficial like water. It is mentioned in "Al-Durr Al-Manthur" that the foam goes away as froth, benefiting no one, while the water remains for people to benefit from. Similarly, falsehood will vanish on the Day of Judgment, and truth will benefit its people. Ibn Qutaybah and Ibn Kathir mentioned that the foam and the dross of metals are parables for falsehood that is cast off by water or the forge and is not benefited from.

As for the mentioned benefit of foam in medical and cosmetic products, it is often sea foam, not torrent foam. Sea foam is formed from the contents of the sea itself, while torrent foam is a mixture of dust and dirt. Even if torrent foam were to have a benefit, the Quran is understood according to what people and rational beings perceive, and the negation of its benefit refers to the absence of benefit in most cases, or in the state that it is in.