You have two versions of something and need to know exactly what changed
That's the situation this tool is built for. Paste the original text on the left and the newer version on the right, then hit Compare. Green lines are additions. Red lines are deletions. If a line changed in the middle, the specific word or character that changed is highlighted within it — so you're not hunting through a whole sentence to find the one thing that moved. This is genuinely faster than reading two documents side by side, which sounds obvious but a lot of people still do it the manual way when they're in a hurry.
Whitespace and case-sensitivity options save a lot of false positives
If you're comparing code that got reformatted, or two versions of a config file where someone changed the indentation, turn on "ignore whitespace." Without it you'll see almost every line flagged as changed when the actual content is identical. Same goes for case — comparing exported CSV data or logs where the case got normalized will produce a wall of red without the ignore-case toggle. These are under the options panel. Worth checking before you assume there are more differences than there actually are. The one thing a diff tool can't tell you is whether a change was intentional — it just shows you what's different. Reading the highlighted output is still on you.