Encoding a value vs a whole URL
When you put text into a single query parameter — a search term or a redirect address — every special character must be encoded, including / ? & and =. When you encode a complete address, those characters must stay as they are so the URL still works. The two modes handle each case.
Reading a messy link
Decoding turns codes like %20 (space), %2F (/) and %26 (&) back into readable characters, which helps when checking tracking links or debugging redirects.