Although every person recites Birkot HaTorah in the morning, the Chachamim established that those called up to the Torah recite the blessings again before and after the reading, so as to instill a feeling of Divine reverence and awe in the heart of the one who is called up, and in the hearts of the listeners.
Originally, the minhag was such that only the first and last people called up to the Torah recited the blessings. The first person called up would recite the first blessing before the Torah reading, and the others called up would not make a blessing. The last person called up would recite the final blessing after the conclusion of the reading.
Subsequently, the Chachamim established that each and every person called up to the Torah would recite the blessings before and after their portion is read. The Chachamim were concerned that perhaps someone would enter the synagogue in the middle of Torah reading and would not have heard the berachah recited by the first person called up, and would think that no berachah is recited before Torah reading. Therefore, they established that each person called would make a blessing before his reading. Furthermore, they were concerned that perhaps a person would leave in the middle of Torah reading. Since he would not hear the last person recite a blessing, he would think that there is no berachah after the reading. Therefore, they established that every person called up would recite the blessing at the end of his individual reading (Megillah 21b). The fact that the Chachamim instituted blessings before and after each reading, demonstrates the importance of Birkot HaTorah (see earlier in this book 10:1).
During the reading, the person who is called up must read each and every word quietly along with the Torah reader. Since he is the one who recited the blessing on the Torah, if he does not read it himself, there is concern that his blessings will have been recited in vain (Shulchan Aruch Orach Chaim 141:2).
In extenuating circumstances, even a person who does not know how to read, or a person who is blind, can be called up to the Torah, despite the fact that it is the opinion of the Shulchan Aruch (139:3) not to call up a person who is incapable of reading the written words along with the Torah reader. Nevertheless, the Rama rules like the lenient opinion, and even in Sephardic congregations it has been customary in extenuating circumstances to act leniently regarding this matter (see Kaf HaChaim 135:16; Yalkut Yosef, part 3, 139:4).