(Mishnah): If a woman had Bi'ah and [afterwards] she saw (became) Nidah, she needs to immerse [before praying or learning Torah]; R. Yehudah exempts. (He exempts also a Zav who saw Keri.)

Question: Does R. Yehudah disagree with the concept of Tevilas Serach (not due to Tum'ah, e.g. Tevilas Ezra for anyone who saw Keri)? Or, is it [only for a Zav or Nidah], because even if he immerses, it does not help (he is still Tamei)?

What is the difference? [These reasons differ about] one who saw Keri [and he is not a Zav]. If you will say that he disagrees with Tevilas Serach, is this Devar Torah?! (There is no need to immerse for Tefilah or Torah!) If it is because even if he immerses, it does not help, here it helps that he immerses. (RIDVAZ, TOSFOS HA'RID)

Answer (Beraisa - R. Yehudah): They enacted this Tevilah only due to Tevilas Serach. Sometimes one goes to immerse, and he remembers an old emission, and he refrains and goes away;

This shows that R. Yehudah does not disagree with Tevilas Serach. Rather, [he exempts a Zav or Nidah] because even if he immerses, it does not help.