ISAC has been now widely recognized as one of most important 6G key enabler. With integrated sensing capability, 6G will be no longer just a platform that connects everything. It will be an intelligent platform that offers both integrated sensing and communications (ISAC) and integrated computing and communications. This platform will provide intelligent services and applications for industries to create greater social value. Bit transmission is not the only function of the 6G network. The physical world will be reconstructed and represented by using the propagation properties of radio waves such as reflection, scattering, refraction, and multipath. The 6G network will serve as a sensing network and 6G terminals will serve as sensing terminals. With network sensing and terminal sensing working in tandem, we can model the physical world covered by the entire network based on 6G. The sensing data extracted from the 6G network will not just be used for modeling the physical world, it will also serve as a big data source and entry for AI learning. Network sensing will enable a new usage scenario beyond communications, covering a series of use cases, including device-based or device-free target positioning, imaging, environment reconstruction and monitoring, and gesture and action recognition. Such use cases will be widely applied to industries, including human-machine coordination, environment reconstruction for smart cities, climate sensing, healthcare, and security detection.