Microsoftsanjibdutta Excel is a functional programming application. It uses what is known as Referential Transparency in FP which is really important in Lambda Calculus which Lambda function is based on. There's no way Simon Peyton Jones et al who brought Lambda to Excel would allow you to do imperative programming type things inside a Lambda function, that would break everything Lambda is it's based on.
There's some good resources on Referential Transparency online if you want to look. Try the Haskell language documentation for a start. TLDR: RT allows us to reason about our code like mathematical equations and also how it prevents entire classes of coding bugs from being created (in imperative programming, something as simple as an accidental coding mistake which increments a variable it shouldn't could erase the database, fire the missiles or whatever…)
