What Adam said... :)
So, if there is a point-in-time that you'd like to have named and available for everyone (i.e. all of the owners in all of the containers) based on the data within the dSource, then a bookmark on the template is the way to do this.
Mind you, a shared bookmark can be created available for every container from any one of the containers, but that would come from the timeflow of the container, which would contains changes from that container, and not from the dSource.
As an example, suppose you want to bookmark the exact end of each fiscal month?
In other words, as a final step of your organization's month-end processing, you create a bookmark that is available for everyone, showing the actual month-end state of the data.
Or perhaps you also want to bookmark the beginning of the month-end processing, so that anyone can easily create a JetStream branch from either point-in-time, for comparison or modification as "what-if" analysis?
Not to imply that use-cases for bookmarks on templates are limited to this, but consider using them to mark points-in-time in which anyone or everyone might have an interest, outside of activities that have taken place within any container.
Just some ideas...
Hope this helps!