Single sign-on (and out) over multiple application types, allowing for centralized login logic and workflow for all of your applications (web, native, mobile, services).
Issue access tokens for APIs for various types of clients, e.g. server to server, web applications, SPAs and native/mobile apps.
Support for external identity providers like (Azure) Active Directory, Google, Facebook etc. This shields your applications from the details of how to connect to these external providers.
The most important part - many aspects of IdentityServer can be customized to fit your needs. Since IdentityServer is a framework and not a boxed product or a SaaS, you can write code to adapt the system the way it makes sense for your scenarios.