That was the argument from expressiveness. This argument presupposes that we have an all-purpose means of combination and abstraction. Interface design is like language design. Therefore, interface designers should know principles of language design. Let's get more specific about the relations between interpretation, compilation, partial evaluation, and optimization.
Expressions allow data to be produced and used without needing a name.
That was Fortran's major advance in expressiveness over fancy assemblers and floating-point interpreters. Maybe I should take a look at MathCad. A statement language can sometimes be extended to an expression language by straightforward generalization. A pitfall of uniform syntax is less margin for error. Static typechecking can help shore that up, as can lint-pickers.