Allo is perhaps one of Google's most controversial products at the moment. It abandoned most of what made Hangouts great, in favor of connecting users by phone numbers rather than email addresses. The app has been slowly regaining some of Hangouts' functionality since release, including a web client. There's just one problem - the web app requires your phone to be turned on with a network connection.
Thankfully, it appears this limitation will be fixed - at some point. Justin Uberti, lead engineer of Duo at Google, said the company is working on addressing the problem:
Yes, we are in the process of migrating the backend system to support completely independent devices.
— Justin Uberti (@juberti) March 8, 2018
At some point, Allo for web will be completely independent of a phone, possibly paving the way for the service to work on tablets and other devices. Like Hangouts did... and Google Talk. As Number Six would say, "All of this has happened before, and it will all happen again."