Nested columns is required for any block builder to allow "non-linear content" Without allowing nested columns each column can only have linear content IE image over top of a paragraph over a button. But you couldn't have three side by side images inside a column or three buttons below an image etc. Currently the builder does not allow this drag and drop insertion of columns inside of columns. Right now you can only dow rows of single columns. See image to help make sense but there are 50% of designs that cannot be achieved with the builder right now.