It would be really great to get some clarity on the intended use of the building blocks from the devs.
If you take a look at the 500 patch notes it states:
Fence foundations should be attachable to regular Foundations and Wedge Foundations
Fixed an issue with foundations where it was possible to create a double layer of protection.
There are several ways, or at least there were, to make walls that took the space of a regular foundation. So I would really like to know exactly the issue they’re addressing.
My common use case for fence foundations to cover normal foundations since I don’t like how the foundations looks alone.
Fence foundations also gives walls on foundation a baseboard. A wall on a foundation will have gaps and a foundation covers that up nicely.
So to me, they serve an important aesthetic purpose.