The Parsha begins by mentioning Har Sinai. The Rambam writes that at Hakhel, the people would listen to the words of the Torah with awe… like on the day the Torah was given at Har Sinai. Hakhel was a re-experiencing of and rededication to the covenant at Sinai.
Some commentaries suggest that the elevated wooden Bimah at Hakhel was a representation of Har Sinai from where the word of Hashem issued forth.
The first Mitzvah in the Parsha is the Mitzvah of Shemittah. The Torah teaches that Hakhel took place “at the end of seven years at the time of Shemittah”, immediately after the Shemittah year. (Some Meforshim learn that Hakhel took lace at the beginning of the seventh year i.e. at the start of Shemittah!)
In Parshas Bechukosai we read the Tochecha - the curses and punishments that would befall the Jewish people if they fail to keep the Torah. One of the passages read by the king at Hakhel were the blessings and curses (in Parshas Ki Savo). Reminding the people of the reward for keeping the Mitzvos and the punishments for failing to do so, was fundamental to the objective of Hakhel.