It will eventually replace the gallery app. It has additional features that include auto uploading/backup of photos, which you do not have to use but some may wish to. Apple does this as well, but they usually either replace an app at the same time or stick with the same app name and just update the app. I think the problem is Google is pushing these out a little early before they are developed enough to actually replace the apps they are intended to eventually replace. Hangouts is another example. I don't currently use the app either, but I can see the benefits.