What's so bad about Eero Bridge mode?

The Eero documentation says that I should connect my Eero router directly to the ISP modem, without using the ISP's router, because in Eero's Bridge Mode I would "lose many advanced features" of Eero.

Which, exactly, advanced features would I lose? 

Of course I could equally ask what benefit my ISP's router provides, and the answer is probably "none" so I probably won't use it.  But I'd still like to know what "advanced features" the Eero router is providing only when connected directly to the modem?