Readable JSON, and the opposite
Machine-generated JSON usually arrives as one unbroken line. Pretty-printing re-indents it so the structure is visible at a glance — nested objects step inward, arrays line up — which is what you want when debugging an API response. Minifying does the reverse, stripping every non-essential byte for when the JSON has to travel somewhere small.
Errors with a location
A browser's built-in parser will tell you something is wrong but rarely where. This tool recovers the character offset from the parse failure and converts it to a line and column, so instead of "unexpected token" you get the precise spot to look — almost always a stray comma or a mismatched quote.