The main reason is being able to see and edit the programming / formulas as overall text, rather than the usual fragmented cell by fragmented cell view. I love named cells and "ranges". I love the concepts of replicating rows or adding a column to a calculation - that's all good. But programming (or editing data) in the small, cell by cell, just doesn't compute.
"Notebook"-style programming comes close. Ideally, I'd prefer if the visualization was handled independently - freely choosing the embedded capabilities if any, or the mountains of visualization software that already exists.
Improv and Spreadsheet 2000 seem to have gone in that direction - but they are still primarily GUI programs.