This package is a part of the Zend Framework 1. Each component was separated and put into its own composer package. Some modifications were made for improved Composer integration. This package can also be found at Packagist.
Size! Zend Framework is very large and contains a huge amount of files (over 72000 files in the main repository!). If you're only using a part of the framework, using the separated packages will greatly reduce the amount of files. This will make setup faster and easier on your disks.
Autoloading! Explicit require_once calls in the source code has been commented out to rely on composer autoloading, this reduces the number of included files to a minimum.
Migration! Zend Framework 2 has been around for a while now, and migrating all your projects takes a lot of time. Using these packages makes it easier to migrate each component separately. Also, some packages doesn't exist in zf2 (such as the zend-search-lucene), now you can continue using that package without requiring the entire framework.
If you're using major parts of the framework, I would recommend checking out the zendframework1 package, which contains the entire framework optimized for composer usage.
Add "zf1/zend-session": "~1.12" to the require section of your composer.json, include the composer autoloader and you're good to go.
Dependencies have been set automatically based on the requirements from the zend framework manual, if you find any broken dependencies please submit an issue.