When to use in keyword with ReadOnlyMemory or ReadOnlySpan?

Question

I was wondering what would be usage of the in keyword with ReadOnlyMemory<T> and ReadOnlySpan<T>.

void Method(ReadOnlyMemory<int> memory)
{
    // Code that modifies memory wont reflect out of this scope.
    memory = memory.Slice(3);
}

void Method(in Memory<int> memory)
{
    // same thing as the above
   memory.Slice(3);
}

void Method(in ReadOnlyMemory<int> memory)
{
    // is there any performance gain here or something?
}

Basically using in or ReadOnly (Span/Memory) isn't similar to const typename & (C++) ?