You can create page layouts. Indesign is used to design brochures, business cards, posters, magazines and books. You can even create eBooks and digital magazines using Indesign.
Can't I do that in Photoshop or Illustrator?
Photoshop would be a bad choice for designing most printed output because it is designed for handling photos. It is not optimised for type flow, layout or multi page documents. Nor is Photoshop capable of creating eBooks or digital magazines. Illustrator can create vector artwork over multiple pages, but it lacks the powerful type handling tools and complex pagination capabilities of Indesign.
How long will it take to learn Indesign?
As always, that depends what you want to do with it. Learning Indesign can take from as little as two hours (for the most basic tasks) up to 20 hours or more (for a fuller range of skills).