I have one general question for which I couldn't find a clear answer. In SF Marketing Cloud Journey Builder, do, or better to say should, we always have a population in place to which we connect attribute groups before actually creating a journey? In other words, is the population an integral part of any regular journey design and what is their true purpose in that sense? Thanks
No, you do not need to have a population in place when creating a journey. It actually states in the documentation that:
Avoid creating more than three populations per account to avoid potential performance issues with your data model.
So it really doesn't make any sense because what if you need to create more than 3 journeys with different audiences -- you can't create more than 3 populations.
Here is some more information on Populations from the Trailhead:
Populations are used to categorize distinct subgroups of contacts. Think of a population as the subset of the master list of people who could enter a journey. Let’s say you work for a car transportation company and you have one master table of contacts, which includes both riders and drivers. You can create two different populations: one population for the drivers, and another for the riders, since separate marketing efforts and data structures are required for each group or population.
If you’re using the most up-to-date Journey Builder functionality, you won’t need to use populations most of the time. Instead, it's best to save populations for specific use cases where you need to create complex queries, such as if your account uses field-level encryption or when you’re using API Entry Sources in Journey Builder.
Populations are a bit of a riddle as they were changed over time and there's never been much documentation. This question thread sheds some light on it. Make sure you read to the last answer to get the latest point.
Summary from thread:
Populations are not required
they actually cause issues in data modelling when setting up attribute sets
they do not cause double billing if the same contact has a consistent key across your instance
not needed at all when working with Salesforce CRM and Marketing Cloud Connect.
Contacts, Leads,PersonAccounts and Users are added to All Contacts automatically as part of the sync process.
To set an email address from a data extension in my data model (other than the entry DE and the All Contacts address) in channel configuration I had to associate this data extension as a population. This use case is a fairly rare, however and pretty much the only one
My current Conclusion: Only use populations when needing a host of different channel addresses besides the default All Contacts address and those cannot be included in your entry data for some reasons.