Minified or messy code is hard to read and hard to review. These formatters restore clean, consistent structure and flag syntax errors along the way.
Formatting makes diffs smaller and mistakes easier to spot. If the input is invalid, the formatter points to the problem, so it doubles as a quick validator before you commit a config file or send an API payload.
Choose indentation that fits your project, and remember that formatting changes whitespace, not meaning. Format both sides of a comparison the same way before diffing to avoid noise.
A messy JSON or XML payload is hard to validate visually, so format it first and only then check types or convert it. The same applies to SQL: a formatted query is much easier to review in a pull request than a one-line statement.