Some content providers -- broadcast networks, cable/satellite network channels, and so on -- offer apps or channels that allow you to watch their content. But, there is sometimes a catch. With some apps/channels, you can only view the content if you authenticate against a TV service provider, such as Comcast, Time-Warner, Mediacom, WOW, Dish, DirecTV, Verizon FIOS, Cablevision, etc.
These channel/apps are not replacements for cable/satellite content, but extensions of your service. They will often allow you to watch their service on another TV that doesn't have a cable box, or to use while traveling. For the traditional broadcast networks, this is useful if you have a locals-only package. For content from History Channel, A&E, and so on, not so much. If you cut the cord, you are not their target audience.
If you have a locals only television package, you can use it to authenticate apps or other Web services of the broadcast networks (ABC, CBS, CW, Fox, NBC, etc).
Some of the apps for these services do allow you to watch limited content without authentication. Some require you to authenticate to watch any content.
While some services, such as HBO and Showtime, do allow you to subscribe to their service without a cable/satellite provider, some don't. It's a new and changing market.