Finding Oracle version is so important in order to maintain the oracle database, apply patches, features or enhancements usage , compatibility usage with other applications. Pl/sql Version is mainly useful if you are working with oracle forms and reports. Tnslistener version can determine the enhancements ( in terms of security) for connectivity issues.
SQL> SELECT banner
2 FROM v$version
3 /
BANNER
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Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production