What was the deal with Haruhi's Endless Eight?
Most likely the last arc of the show was originally planned to adapt The Disappearance of Suzumiya Haruhi, but the producers changed their mind well into production, and decided to make it into a movie instead. As a result they had to fill in a seven episodes gap that they already bought the timeslot for, thus this gimmick was born.

They Endless Eight chapter in the novel is only about the very last iteration where Kyon figures out the solution.

The point of Endless Eight was to provide a reason for Nagato to do what she did in Disappearance, being the only one to remember the over 15000 iterations.