Image Credit: WABetaInfo
This preview feature can potentially help users save time and storage space. By providing a thumbnail of the file that has been sent, the recipient can make an educated decision of whether to open and download the file. It can also help when trying to find a file that had been sent before, giving users a quick way to scan through messages, so only the correct file is opened. Especially with larger files, a quick preview could be a lifesaver.
Right now, the document preview feature is in development for Android users participating in the Google Play Beta Program. Unfortunately, as I have come to realize, this is a very difficult beta to get into as it is always full. However, WhatsApp has a good history of not having its users wait too long before rolling out features that are in beta. Hopefully this rolls out widely very soon.