Comparing two versions of a piece of text by reading both is slow and unreliable, and reading them side by side is barely better. The problem is not that differences are hard to spot individually. It is that the eye adapts to whatever it is reading, so an inserted paragraph in the middle of a familiar document can be almost invisible unless it is marked.
What the comparison shows
The two texts go in side by side and the result highlights what changed. Lines present in one and not the other are marked as added or removed, and lines that were edited are marked as changed. Because the differences are marked rather than merely present, the eye is directed to them instead of having to notice them.
Where it earns its keep
The most useful application is confirming that a change you made landed as intended, particularly when several people edited the same thing. It also solves a common problem with copying between systems, where text picked up from a document, an email and a web page can pick up differences in spacing or punctuation that are easy to introduce and hard to notice.
It is also the quickest way to find out how much two versions actually differ, which is useful before deciding whether a merge is worth the effort.
Line endings and invisible characters
Plenty of confusing comparisons come down to something you cannot see. Text copied from Windows may carry different line breaks than text from a Unix system. A stray trailing space at the end of a line will show as a difference even though nothing appears different on screen.
If a comparison shows a change on lines that look identical, this is the most likely explanation, and it is a reminder to paste carefully rather than a fault in the comparison.
Everything stays local
Because the comparison runs in the browser, you can check a contract or a draft without sending it anywhere. For text that is confidential, or simply not ready to be seen, that removes the reason to use a hosted tool.
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Reading a comparison properly
A comparison is easy to misread if you treat every marked line as equally significant. Lines that were added are usually the ones you care about, and lines that were removed are often the ones you will miss, because nothing in the new version draws attention to what disappeared.
Read the removals first. Deleted content is the easiest thing to overlook and frequently the most consequential, particularly when a change is meant to be additive and something was dropped along the way without anyone intending it.
Then check the size of each change. A single altered line in a document of hundreds is usually a wording fix. A large block replaced at once is worth reading properly rather than scanning, because that is where meaning tends to shift.
Where the two versions differ mainly in whitespace, the differences are rarely substantive, and a comparison full of them usually means something reformatted the text on the way through rather than edited it. That is worth knowing before you spend time reviewing every mark.
For a genuine rewrite, comparing the two versions after the fact is less useful than it sounds, because by then you no longer remember what you intended. Comparing before you commit to a change catches far more.