The fact that you don't understand when to use InDesign suggests that your workflow probably doesn't include documents where you would need to use InDesign. InDesign is a program designed for creating documents that combine text and images, often over multiple pages, and usually with a significant amount of text. It provides many tools to ensure consistency over multiple pages, and some really sophisticated tools for manipulating large amounts of text.
Although Illustrator includes some of the same tools for controlling text layout, the level of control is much reduced.
As someone who is familiar with Illustrator, a good way to get the feel of how they are different is to open up InDesign, and use the drawing tools that are available in that program. Although there are overlapping tools that function in much the same way that they do in Illustrator, you will find that the more nuanced tweaks you can make in Illustrator are simply not available in InDesign. The text control and layout functionality available in Illustrator is similarly basic compared to what can be done in InDesign.
But the real key is to use whatever you are most comfortable with until you begin to get frustrated with the limitations of the program that you are using. If you are suddenly asked to create longer documents, with much more text than images, I am pretty sure you will become frustrated with trying to create them in Illustrator, and will begin to understand when it is time to switch over to InDesign.