One aspect about applications that developers are very concerned about is authentication. ASP.NET framework has been providing authentication in 3 ways for years - Windows Authentication, Passport Authentication, and Forms Authentication. Later, the ASP.NET Membership Provider model was released which simplified the process. But the web evolved again over time, and the provider model failed to meet the changing needs. 

It needed a good replacement. Though Microsoft released more versions of the membership systems, they were still either associated with the Forms Authentication module or were useful only for relational persistent storage. Even businesses providing ASP.NET development services had trouble keeping up with changing trends using just the existing models. Eventually, to catch up with the evolving trends of web applications, ASP.NET Identity was released - a replacement to the existing model.

ASP.NET Identity

A system built from scratch to address most, if not all, issues associated with the authentication aspect of web application development, the ASP.NET Identity works across systems like MVC, Web API, WebForms, WebPages, Smartphone applications, hybrid systems etc.

ASP.NET Identity is:

• Highly extensible.
  
• Compatible with External Login Providers or OAuth Providers through middleware components.
  
• OWIN Compliant
  
• Supports all databases that support Entity Framework
  
• Makes unit testing of applications easier
  
• Compatible with Azure Active Directory
  
• Supports Role-based and Claims-based Identity System
  
• Delivers patches and bug fixes easily to developers as it is delivered as Nuget packages

Conclusion

You can still use Forms Authentication for the same purpose of ASP.NET Identity. However, you will be able to run it only on IIS, and it depends on the System.Web assembly. This also makes it non-OWIN compliant, while ASP.NET Identity is fully OWIN compliant. Because OWIN compliance is vital at present and for the future, ASP.NET Identity is the recommended microsoft technologies solution for building new systems that meet the present trends.