Indenting your text on the left and/or right is appropriate in the following situations:
Figure 9.14. A table of contents using indents.
Text insets are an inner margin on your text frame. They give you the same result as applying left and/or right indents, but might be more convenient if you are working with type in a framed or colored rectangle.
If you use a rounded rectangle or any kind of custom text frame shape you will only be able to specify a single dimension for the text inset.
Figure 9.15. Text Frame Options.
Figure 9.16. A sidebar text frame using Inset Spacing.