Lightboxes vs Layers

I second what Andrew says. When you need the same content to be available from anywhere in the course, it makes sense to lightbox it. That avoids having to edit the content in multiple locations. 

Andrew's example is a good one: allowing the user to easily review the slide(s) related to a question. 

Another good use is to lightbox content that might be useful throughout the course via a Player tab. For example, that could be a custom glossary or resources slide. Or a Help slide. This post has more info about this option: https://community.articulate.com/discussions/articulate-storyline/tip-create-custom-player-tabs-for-your-resources-glossary-and-more 

Historical footnote: In the early days of Storyline, using a lightbox instead of a layer was the only way to prevent the user from interacting with the Player when that content was open. Now, however, one can use Dialog layers. Those, like lightboxes, cover the Player.