It sounds odd at first that we need to install sharepoint on Report Server. The installation actually makes RS as one of SP WFEs even though it normally doesn't actually handle any sharepoint web requests (no AAM configuration for RS server)
The reason that SSRS need sharepoint is that RS extension use sharepoint object model to do security checking (see
here for details), and SP OM can only be available on a sharepoint server.
The fact that SSRS use OM to check permission essentially makes it possible to use "Trusted Account" when configuring report server from SharePoint Central Admin:
With "Trusted Account" setting, you don't need to enable kerberos for either sharepoint site or RS web server. The downside however is, you won't be able to get a security trimmed reports. Data access is through either sql authentication or through a credential mapping.
