It is not for Tsitsit but , but it is a part of the Shema, the set of the 3 paragraphs. I
See Berachot 21a
Rab Judah said: If a man is in doubt whether he has recited the Shema', he need not recite it again. If he I in doubt whether he has said 'true and firm', or not, he should say it again. What is the reason? - the recital of the Shema' is ordained ...
In the Yerushalmi Brachot 8:1 (56b) (as well as in Yerushalmi Pesachim 10:2 (69b)), we read:
א"ר יוסי בר' נהיגין תמן במקום שאין יין שליח ציבור עובר לפני התיבה ואומר ברכה אחת מעין שבע וחותם במקדש ישראל ואת יום השבת
Said R. Yosé b. Rabbi, “This is the custom [in Babylonia] in a
place where they have no wine [available for the recitation of the
There are a number of authorities who allow taking in the second day of yom tov early. From my quick review of the sources they do not discuss the issue of tosefes.
See Aruch Hashulchan Orach Chaim 668:6 and Yom Tov Sheni Kehilchato p. 37.
The Taz (Orach Chaim 489:10) was against this practice but see response in Sha’ar HaTziyun 668:11