Eren could use Reen to extract Julie's memories and share them with him. At least that's what he was planning before using any other means.
Reen's powers were peculiar in a lot of ways. She wasn't a battle-oriented demon beast. But her other distinguishing abilities made up for that lack of traits.
When Reen ate her prey, she processed their memories as well. But she used to ignore most of those fragmented memories because they weren't compatible with her. Her lack of sufficient intelligence was also a roadblock.
All she could process were some bits and pieces that didn't make sense in her head.
But all that changed with her new transformation, increasing her intelligence in the process. At least that's what Eren had conjured. The beast contract spell gave Reen the ability to process the memories of her prey better.
