Tomcat can use the Apache Portable Runtime to provide superior
scalability, performance, and better integration with native server
technologies. The Apache Portable Runtime is a highly portable library
that is at the heart of Apache HTTP Server 2.x. APR has many uses,
including access to advanced IO functionality (such as sendfile, epoll
and OpenSSL), OS level functionality (random number generation, system
status, etc), and native process handling (shared memory, NT pipes and
Unix sockets).

These features allows making Tomcat a general purpose webserver, will
enable much better integration with other native web technologies, and
overall make Java much more viable as a full fledged webserver platform
rather than simply a backend focused technology.