I am looking at code from Microsoft and I noticed the following:

An optional parameter std::vector<T>* vOut is passed into a function.
The function itself uses a separate local std::vector<T> result for the optional result.

At the end of the function, they do the following to pass the result to the caller:

if(vOut) {
    std::swap(*vOut,result);
}

why would someone use a std::swap here instead of a move-assignment like
*vOut = std::move(result)?

Here is an example