The closing daf of Massekhet Nidda, indeed the closing daf of the Babylonian Talmud, ends with the following statement:
The school of Eliyahu teaches: Whoever repeats words of Jewish law every day may rest assured that he will have a share of the World-to-Come, for it is said (Ḥabakkuk 3:6), halikhot olam lo, literally, “His ways are eternal”; read not halikhot (his ways) but halakhot (Jewish law).
While Rashi and others have this statement as part of the Gemara, it does not appear in many manuscripts, and may be a later addition. Tosafot explains its appearance based on a Gemara in Massekhet Berakhot (daf 31a) that teaches that just as we find that the prophetic books close on a positive note of praise and comfort, it is appropriate for the Talmud to close in a similar manner.
Rabbi David Luria suggests that this statement is particularly important here, since the Mishnayot of the Order of Ṭohorot continue, and the editors of the Gemara feared that students would conclude their study with Massekhet Nidda. The statement encouraging the daily study of halakhot, i.e. of Mishna, means to encourage completion of the other tractates. According to the Maharsha this statement was included in order to emphasize to the individual who completes Massekhet Nidda that even though the accepted stringency instituted by Rabbi Zeira (see above, daf 66) renders many of the laws of the tractate moot, nevertheless there is an independent value of the regular study of these laws.
The Arukh LaNer points out that the final Mishna at end of the Massekhet Oktzin, which is the very last tractate of the Mishna, has a parallel conclusion, describing the great reward that is due to the individual who studies Torah, based on the passage in Mishlei (8:21). At the conclusion of the study of all the laws of the Mishna, it is appropriate to be told of the spiritual reward that accompanies such endeavors. Similarly, at the end of the study of the Talmud, a statement attesting to the ultimate reward of a place in the World-to-Come is certainly in order.