What does this extra pair of parentheses do?

While looking into a piece of code, I saw this line:

if ((b = a)) { /* statements */ }

Reading the context, I know its intention is like this:

b = a;
if (b != 0) { /* statements */ }

I know the two lines above can be simplified into the first code block, but why are the two pair of parentheses? Wouldn't that seem redundant? I think this is totally OK:

if (b = a) { /* statements */ }