It’s the conduit through which a lot of Legends stuff is somewhat canonized, basically. Things like Delta Squad or Quinlan Vos were in the show as references to the larger EU at the time. Without The Clone Wars remaining canon, certain things and characters would only be Legends now. It’s the middle of a Venn Diagram.

I kind of find it ironic given how TCW used to get flack for ignoring the EU to do its own thing. Now it’s the only canon media keeping a lot of that EU a part of the main lore.