Having code and actual application data allows people without programming experience to build data types and simple to average functions and debug errors in the business logic, whereas with classic code they can't get pass the parser to get their poorly written syntax understood by the compiler.
Developers tend to think that writing simple code makes application development easy, but actually learning the syntax and runtime model of classic imperative code is usually the hardest part to newcomers.
The functional-reactive execution of the spreadsheet together with the intermixing of data and code provides a simple mental model of instant evaluation and exploration, similar to the benefits a REPL provides to a seasoned developer, minus the syntax errors and deep nested structures.
Notebook-style development environments like observablehq.org are almost there (specially those with a spreadsheet-like semantics), though it still keeps too harsh a separation between the code environment and the "runtime values" environment - i.e. its cells are "too large" when compared to spreadsheet cells.