Hello! What is the historical reason for having Maftir? I can understand why we have Haftarah, due to the many years that we could not read from the Torah. But why is there a need to recite a portion of the 7th aliyah over again? Thanks!

ב"ה Because we read the Haftara is the very reason for reading over a small part of the seventh Aliya. The Gemara in Megilla 23a, deals with your question. Since we read the Haftara from the Nevi'vim, Chazal wanted to show the different levels of holiness between the reading of the Torah and reading the Navi. They did not want it to look like that if someone reads from the Torah or reads from the Navi it is all the same. So first, the person reading the Haftara reads from the Torah to show respect for the Torah. Meaning, the sole purpose of this Aliya is to show respect for the Torah and it is not and inherent obligation like the other seven aliyot and it can't be counted as one of the seven. There are Rishonim, (Tosfot, Rosh and Rashba however, who hold that we need not to repeat that which already has been read, but the reading can be divided into 8 portions and they say that this was the custom in the time of the Tana'im. However, since the custom became to say kaddish between the 7 aliyot and the maftir, Chazal did not want to interrupt the reading of the Torah before the entire Parsha was read. So therefore, the custom is today the way it is. Chag Sam'each