Our Gemara on Amud Beis noted that though the signs of a kosher fish are fins and scales, in fact every fish that has scales also has fins. The Gemara then asks, if so, why did the Torah bother to write fins and scales? Why not just write every kosher fish has scales and that would be sufficient? The Gemara answers:

“It is written: ‘…To make Torah great and glorious’ (Isaiah 42:21). God consequently expanded some aspects of the Torah more than strictly necessary.”

The simple explanation is as Tosafos states, the Torah provided multiple sources, which implies that the Torah may provide an explanation that is more than strictly necessary. This is apparently a rule, but there is no stated reason as to why. Presumably, the Torah is assuming that it may not always be easy or clear to deduce a matter, so the divine wisdom and will deemed certain seemingly superfluous indicators to be of value.

I will add, the idea of fins might have a lesson or truth locked inside that we are invited to look at, now that it too is a formal sign, even if not necessary because we can determine kashrus from scales alone. For example, the Shalah (Torah Sheboksav, Derech Chaim, Shemini 5) offers an explanation for why these species are not kosher. Most are familiar with the obvious concept that the non-kosher beasts and birds are predators as opposed to the more passive kosher cattle and birds. Shalah extends this idea to the fish and grasshoppers as well. The kosher grasshoppers hop on knee joints according to the verse (Vayikra 11:21), which indicates that they are non-predators and are equipped to run away swiftly. The kosher fish have protective scales and fins to run away and guard against predators as well. The Shalah uses this as an indicator of the sensitivity of the Jewish soul. The mere consumption of a predatory animal can somehow increase a predatory behavior within the person who consumes it.

Mei Hashiloach (II, Shemini 7) sees a self-referential hint within the law itself. Scales represent the protective exterior of the fish, a defensive status quo. Fins represent the ability to ambulate and travel. The metaphoric scales of the Torah are the truths that are intrinsic and should be known. The fins are the ways to navigate the “sea of the Talmud” and know more. Though one could have known kosher fish from their scales, God also taught fins because it still is a kosher sign and there is some Torah idea hidden within it that now has become easier to see. God gave fins to navigate and find out even more, beyond the given and deductive truths.