It is quite common to see header/footer lines in real world data set. So skipping first/last N lines in "create external table" DDL can be useful feature for Tajo users. In this way, user don't need additional processing of data which generated by other application with a header or footer and directly use the file for table operations.
cf. Same feature added in Hive 0.13 : https://issues.apache.org/jira/browse/HIVE-5795