When you would want it

In languages where indentation decides what belongs to what - Python especially - the lines are the difference between reading the structure and guessing it.

Where to find it

All Features → Indent guides
Turn on or off
Settings

What you get out of it

You can see which lines belong together without selecting anything.
The active-block highlight answers 'which loop am I in' continuously.

How to use it, step by step

On by default - faint vertical lines mark each indentation level.
The guide for the block your cursor is in highlights brighter than the rest.
In brace languages they follow the structure; in Python and friends they are the structure.

Why this beats the usual way

Guides that show all levels equally just add visual noise. Highlighting the enclosing block turns guides from decoration into orientation.

Find it in Universal Search

Indent guides · Editing