Why "Yeast Extract"

Yeast extracts provide umami, so it's a flavor enhancer. In the burger joint context, I imagine you might think it makes things taste a bit more meaty.

The most famous yeast products are things like marmite and vegemite, but it doesn't have to be that intense. And since it's pretty easy to produce and can be dried into a powder, it's a common ingredient. You've probably had it in all kinds of commercially produced food without even knowing it.

Yeast extract contains a high content of glutamic acid, which, together with salt, forms MSG - hence the Umami taste. It's a flavour enhancer.

As MSG has been the focus of many health / nutrition scares and blamed for "everything" from cancer to obesity, using yeast extract allows a manufacturer to avoid writing "MSG" and still have it in the food. -> Even the claims "no MSG added", "no artificial flavors" or "all natural flavours" are valid.