Understanding Immutability in Effects and Its Benefits

This might sound really dumb but i would really appreciate if someone could help me with this, I needed some more clarity as to why we call Effects "immutable" and how does that differ from usual non-effect code. What gains do we receive when using this "immutability" of effect over code that isn't.

Does this immutability basically infer that effects are self contained, lazily executed, instruction sets which won't mutate any global state on their own unless ran explicitly? or... ? :/
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