Functional Programming, "no side effects" and so on...

Has anyone got a link to an explanation of the philosophy of functional
programming in general/XSLT specifically -- the "no side effects" idea
and so on?

I think I've got over the initial frustrations of XSLT not being like
other languages in that you can't increment a variable and so on, but
I'd like to understand why that's considered a good thing in terms of a
bigger picture, what benefits it provides, etc.

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