Why does Volatile.Read take ref parameter?

The implementation of Volatile.Read merely inserts a memory barrier after the read:

public static int Read(ref int location)
{
    var value = location;
    Thread.MemoryBarrier();
    return value;
}

Thus, usage of the method like so...

return Volatile.Read(ref _a) + Volatile.Read(ref _b);

...would be equivalent to:

var a = _a;
Thread.MemoryBarrier();
var b = _b;
Thread.MemoryBarrier();
return a + b;

Given the above, wouldn't the resulting behavior be identical if the parameter was not a ref?

public static int Read(int value)
{
    Thread.MemoryBarrier();
    return value;
}

I guess that the ref parameter was used simply to prevent programmers from passing things other than variables, such as Volatile.Read(2 + 3). Can anyone see any other reason for the ref when variables are passed in?