Surprisingly, the most obvious answer is missing: trailing whitespace can and will produce difficult to find bugs.
File "demo.py", line 1
SyntaxError: EOL while scanning string literal
which is somehow cryptic and difficult to solve if the editor is not configured to display whitespace characters.
While syntax highlight can help avoiding such cases, it's even easier to not having the issue in the first place by not letting whitespace at the end of the lines. This is why some style checkers will raise a warning when encountering trailing whitespace, and some editors will trim them automatically.
Illustration: syntax highlight can help avoiding trailing whitespace in situations where it can lead to bugs, but don't rely just on it.
Another context, briefly mentioned in a previous answer, is data stored in files.
For instance, CSV files which contain trailing whitespace may cause data inconsistency which is also very difficult to detect: standards-compliant parsers will trim the whitespace (the standard indicates that leading or trailing whitespace is irrelevant, unless delimited with double-quotes), but some parsers may misbehave and keep the whitespace as a part of a value.
Other custom formats may specifically consider that whitespace is the part of the value, leading to consistent but still difficult to debug situations.