Cabinet meetings are pretty rare nowadays. Whatever they want to do, it usually doesn't involve getting 15-25 people with very different portfolios together in one room.
Warren could provide Hillary with a public face for her administration that would be attractive to the Democrat base. Clinton probably wouldn't actually do anything Warren suggested, but Liz might be slow in catching on to how things were.
A lot of politicians are flattered by simply being in the loop. Biden may be the kind of person who treasures and cherishes the few moments when he had any influence over anything at all, and forgets about all the matters that he had no say in. I don't know if Warren would be the same way or not.
Clearly Warren suffers from a lack of reasonable curiosity about subjects that should interest her (economics, for example). But he is an intelligent woman with an obvious hunger for power. I think she understands Mrs. Clinton quite well.