A trailing slash is one character, and it is the difference between two URLs that most systems treat as entirely separate addresses. Normalising a list so every URL ends the same way removes an entire category of confusion.
Why the slash is not cosmetic
/about and /about/ can serve the same page, redirect to one another, or serve two different pages. Which of those is true depends on server configuration. What is always true is that analytics, crawlers and redirect maps treat them as distinct strings.
What the tool does
It adds a trailing slash to every line that does not already have one, and leaves the ones that do alone. That second half matters — a blind append produces /about//, which is a third distinct URL.
When to normalise
Before comparing two lists, before building a redirect map, and before deduplicating. Comparing un-normalised URL lists produces false differences that cost real time to chase down.
Try it: Add Trailing Slash to URLs on SeoWolf's Notepad.