The discovery of the notebook and the evidence that Titans might be sentient changed the Scouts' approach, finally convincing Erwin to allow Titan capture missions. Watch Information
The episode follows two parallel timelines: the present search by the Scout Regiment and the past account written in a diary. Shingeki no Kyojin OVA - Episode 1
This was the first time in the series a "mindless" Titan was shown to have the capacity for speech and complex emotion (reverence). The discovery of the notebook and the evidence
Confused and terrified, Ilse attempted to question the Titan about its origins. Her yelling triggered the Titan's aggression, leading it to tear its own face before eventually killing her. She continued writing her observations until her final breath. Key Takeaways & Significance Shingeki no Kyojin OVA - Episode 1