Currently a developer can choose to include a Javascript library as part of their widgets installation files. However this adds bloat and such libraries may not be fully compatible with the S60 browser.
jQuery will be included in the WRT development kit which is used to create widgets for the S60 platform. Standardising around jQuery should encourage widget developers to use the built in library rather than including one themselves. It also likely that a thorough testing program will take place to ensure 100% compatibility between the library and the S60 browser.
jQuery should help make it easier for developers to create more complex widgets; in general in reduces the amount of Javascript that a developer needs to write in order to carry out a particular task. Including jQuery in WRT also provides web developers with a clear sign that they can work with a familiar environment to develop browser based applications on S60 phones.
Here's an extract from the blog post:
Nokia is looking to use jQuery to develop applications for their WebKit-based Web Run-Time. The run-time is a stripped-down browser rendering engine that allows for easy, but powerful, application development. This means that jQuery will be distributed on all Nokia phones that include the web run-time.