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.
Although I’m a big fan of the old EU I do actually prefer TCW as a new canon series, especially now that we have Rebels and season 7 (Legends TCW will always be unfinished and it was a rough ft to begin with), but I’m still fascinated by the fact that they haven’t bothered to create any new canon introductions to Grevious and Ventress. Middle of a venn-diagram as you put it is a pretty good way to describe the show overall, and it does seem pretty fitting as the last Legends entry and the first new canon entry as it was the last SW-thing that Lucas worked on.