thx for the reply.
Yes, SSRS needs a database but when selecting a "new" install, I can just as easily select only the Reporting Services component and then configure SSRS to create its databases on the SQL instance already installed. You don't need to select both the Reporting Services and Database Engine together. In fact, in most cases we install the Reporting Services component (without the engine) on a separate server (this means an extra license though) and use an existing clustered SQL instance to host the Reporting Services databases. This way we also have high availability (at least for the databases, later we can scale out the SSRS component if need be by adding more servers). It's also the preferred way because of security reasons (engine = backend, report server = frontend).
But some time ago a customer, already running a SQL instance for years, needed a Reporting Server. They asked to have its SSRS service running on that same host to avoid the extra license. When installing I just wondered what the difference is when adding SSRS to the existing installation as an extra feature/component as opposed to installing it as a new installation (so just selecting the SSRS component and then configure it to create its databases on the installation's database engine that was already present).
One difference that comes to mind is this: when you add SSRS as an extra component to an already existing db engine installation, you will end up with the same edition. Say for example you installed a SQL instance using Standard Edition, your added SSRS component will also be Standard Edition. If instead, in this scenario, you would like Enterprise edition for SSRS because of certain features, you have no other way than to upgrade the existing installation and add SSRS (before or after the upgrade) or to add SSRS as a new installation (Enterprise that be). In the last situation you would thus have a SQL Standard Edition database engine and a RS Enterprise Edition instance running on the same host (with the RS databases hosted in the Standard Edition's db engine).
I performed several tests in the past but I was wondering if someone encountered any critical issues with either approach. Something that doesn't make me regret my choice later.