JDBC is an access API. PgJDBC provides an implementation of the service provider side of that API so that applications that use JDBC can use it to talk to PostgreSQL. It's "glue" code that connects JDBC to PostgreSQL.
In the same way, psqlODBC is "glue" code to connect ODBC API users to PostgreSQL.
Also, JDBC is developed under the BSD license. PgODBC is develop under the GPL License.