One of the Parshios read by the king at Hakhel is the passage of Aser Te’aser in Parshas Re’eh. This passage discusses the Mitzvah of separating Maasros from the produce of the field, including Maaser Sheini and Maaser Ani.
The king would then skip to the Parsha of Vidui Maaser in Ki Savo, which shares a similar theme. (According to the Rambam, the king would not skip to Ki Savo, but would continue reading in order).
The Parshios at Hakhel, such as Devarim, Shema, Vehaya and the blessings and curses, were read because they serve to inspire the people in the fear of Heaven and observance of the Mitzvos. The passage of the king was read because Hakhel is a Mitzvah performed by the king.
But why were the passages of Maaser read?
Hakhel came immediately after the Shemittah year (which is also mentioned in Parshas Re’eh). During Shemittah, when no work was done in the field and the produce was Hefker, there was no separation of Terumah and Maaser. The Parshios of Maaser were read at Hakhel as the Jewish people would go back to their fields, so that they would not forget the laws of Maaser.
