[brundolf]

It might be because nested functions are closures, which are optionally-named, optionally-typed, and can capture values, unlike top-level declarations. It's not unusual for languages to have a special closure/lambda syntax (whether that's really necessary or good is another question, but there's a lot of precedent)

[garganzol]

Any function is a closure. Top-level function declarations can capture static variables and type members. Nested functions can capture static variables, type members, and local variables of a declaring function. Difference is negligible.

[brundolf]

Conceptually, as a high-level user of a typical language. But there can be nuanced differences in the implementation and/or the semantics, depending on the language. I don't know if that's true here, but it's certainly plausible.