It is the rule of thumb to use API_EXPORT with module API functions.

To be honest, I don't see strict reason to use this macro except for
unification with other module API functions.

It adds 'extern', which is default for functions.

It adds __attribute__((visibility("default"))), but symbols to be
exported are listed in extra/exports or src/export.h (depending of
tarantool version, see [1]).

It adds __attribute__((nothrow)), which maybe allows a compiler to
produce more optimized code and also catch an obvious problem and emit a
warning. I don't know.

So the reason for me is unification.

Part of #5273

[1]: 2.5.0-42-g03790ac55 ('cmake: remove dynamic-list linker option')
[2]: 1.6.8-71-g55605c5c9 ('Add __attribute__((nothrow)) to API_EXPORT macro')