I could be wrong, but I believe it is to add emphasis to the fact that you are coming because it is today, rather than you are just coming today.
I think 今日にくる is the xにくる grammar, meaning you are coming for x. So, 今日にくる来れれば would be "if you come with the purpose of coming because it is today. "
Maybe you are speaking to someone that wouldn’t have you over usually, but today is special for some reason, like it’s their mom’s birthday, and they are emphasizing that if you are coming because today is today (a special day), then you can come, but you can’t come if you are just coming to come over as though it were any other day.
I’m confusing myself a little explaining this, but I think that’s what’s happening, but again, I could be wrong.