As with the other food laws, observant Jews keep this prohibition as an act of obedience and sanctification rather than for a stated reason, and it is held across the observant denominations. The clear, physical test, fins and scales, makes the category easy to apply and hard to blur, which is part of its function in marking a distinct way of life. It belongs with the prohibitions of pork and of forbidden birds and insects as part of the single fabric of dietary holiness the Torah lays out.
The rule produces an instructive contrast with Catholic fasting practice, where all seafood, shellfish included, counts as permitted non-meat food during Lenten abstinence, since the Western Christian category of forbidden meat was built around warm-blooded land animals rather than around the biblical categories of clean and unclean creatures Leviticus sets out; the same shellfish a Catholic might specifically choose to eat on a Friday in Lent falls under an entirely separate and, for Jews, prohibitive classification, illustrating how two traditions can build very different dietary systems from different starting principles even while both restricting food for religious reasons. Between the Torah's own text and modern kosher certification stands Maimonides' twelfth-century law code, the Mishneh Torah, whose section on forbidden foods, Hilchot Ma'akhalot Assurot, gives the fins-and-scales rule its classical, systematic legal statement, the layer later rabbinic authorities and certifying agencies build directly upon.