My understanding is that the database engine feature of sql server enables us to create databases.

In the SSIS documentation, <https://learn.microsoft.com/en-us/sql/integration-services/install-windows/install-integration-services?view=sql-server-ver15#install-integration-services>, it is mentioned that:

> 
> On the Feature Selection page, under Shared Features, select
> Integration Services.
> 
> 
> Under Instance Features, **optionally** select Database Engine
> Services to host the SSIS Catalog database, SSISDB, to store, manage,
> run, and monitor SSIS packages.
> 
> 
> 

What is the purpose that one would not select the database engine service while installing integration services?

It is clear from the documentation quote above that - if we do not select the database engine, then we cannot host the SSIS catalog db (ssisdb). So what is the purpose of such an install?