location-based-mode. Why do we set Home Location at the PF level?

SImple. because that's where it needs to be from the point of view of the Online part of BRM.

When your customer picks a location we determine which Products are relevant to that search.  We show any and all categories containing products with a matching home location.

When you subsequently open a Category, you see your inventory organised according to Product Family.

These are the rows that are brought back based on their Home Location.

You can see this clearly from the image below:

Products Lines represent sizes within a Product Family so it wouldn't make any sense to have location that low down and Online wouldn't work.

If you find yourself needing more granular locations you need to create a new Product Family.

For example two identical bikes, one at each location, you're going to have to have a PF for each location.