What is the point of installing integration services without database engine?

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

In the SSIS documentation, https://docs.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?