To Define Shapes
The design on the left is confusing. Everything runs together. You can focus on little bits here and there but it's hard to get an overall picture.
Add some plain sashing and you get the design on the right. It is easy to know what the block is. The confusion is gone and the design is more satisfying. It's clearer and calmer.
This second example isn't as confusing. You can see the block easy enough. But it feels cramped.
Add some sashing and it is more spacious, more orderly and airier. It feels friendlier.