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"You are in a maze of twisty little passages, all alike."

Personally I use it to map my presentation into chapters. It allows me to better know where I'm at while presenting, and allows people reading my slides to get an idea of how it's laid out inside my brain.

It was just used by a colleague to organise his slides into:

Topic ▼ Code example ▼ What that code renders into

The last slide was actually rendered live from the slide before it. Absolutely badass.

Probably to provide the possibility of not drilling down into details so you can change the presentation based on the audience.