Why was Omar from The Wire written as gay?

Like many fans of The Wire, Omar is one of my favorite characters. But why did they decide to write him as gay? Is it of any significance? Does it really add anything to the show? Like if he wasn't gay, would he be a different character at all? I have been thinking about it, and at first I thought it does play a role in his persona, but recently I've changed my opinion. What do you think?