By default, string comparison fails if two strings are identical except one string contains extra spaces. For example, one field value in a test is 'Data Validation' and the other field value is 'Data Validation ' (with three blank spaces after the last character). If you do not trim trailing spaces, the test produces a bad record. If you trim trailing spaces, the comparison passes because the extra spaces are ignored.
You might want to trim trailing spaces when the CHAR datatype, which pads a value entered with spaces to the right out to the length of the field, is used. A field of CHAR(20) compared to CHAR(30) fails, even if both fields have the same value, unless you trim the trailing spaces.
Trim Trailing Spaces
option if there are spaces after the value entered in a field that should be ignored in the comparison. This option is disabled by default.