Also, Google plan on personalizing the feature which is clearly an edge over Siri as it is in a closed environment.
The project was a part of the earlier leaked Majel project from Google, which was named after Majel Barrett-Roddenberry, the wife of Star Trek creator Gene Roddenberry; however, now it is being taken up by the Android team as an attempt to expand the existing Voice Actions. Its not like Google is completely new in the field of voice control. Android has some pretty decent voice assisted features.
It is important for Google also because Apple is steaming ahead with the Siri and introducing it in the Apple TV. Google can't just sit and watch Apple take the market in its grab. Although, there is some respite for Google as Apple have not used Siri in the recently announced 'new i-Pad' which was supposed to have Siri pre-installed in it.
Google's Assistant is presently a work under development and might be released by the end of 2012 along with, maybe, a new version of Android. Until, then , nothing much can be said with complete certainty.