Shulchan Aruch rules (O.C. 62:5) that the chazzan should say the possuk “Shema Yisrael…” out loud, so that the tzibur should join in together and accept ohl malchus shamayim in unison.
That itself is one point: That the chazzan say the possuk aloud, for the reason given.
The Bais Yoseph states that this implies that that even someone who has already said shema should repeat this possuk together with the tzibbur. And certainly one who has not yet said shema should say the possuk now. And Bi’ur Halacha cautions that such a person should have in mind not to be yotzei the mitzvah now.
סימן סב – מי שלא דקדק בקריאת שמע או לא השמיע לאזנו, ובו ה’ סעיפים
ה צָרִיךְ שְׁלִיחַ צִבּוּר לְהַשְׁמִיעַ קוֹלוֹ בִּשְׁמַע יִשְׂרָאֵל, כְּדֵי שֶׁיִּשְׁמְעוּ הַקָּהָל, וְיַמְלִיכוּ שֵׁם שָׁמַיִם בְּיַחַד: