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.