Taken from here:

We feel that all software should have a changelog that details, at a high level, what changes have been made in each version so that the user can make an informed decision about when to upgrade and how much testing they should do with their site.

In order to make this an easy and open communication channel we have added support for a Changelog section in the plugins readme.txt file.  This changelog information is then displayed as a separate tab in the plugin directory and also in the back end of your WordPress blog when you view the details on a new version of a plugin.

I personally agree with this decision of the Automattic and the WordPress team because I have found some annoying plugins which don’t state why a plugin is updated. This should solve that problem rather nicely!