The problem with leaving a big gap ahead of you is that other cars may weave in to try and get ahead of the traffic... at the expense of slowing down your commute since there are now more cars ahead of you and you haven't moved an inch.
Yeah, but then I'm helping thin out the larger congestion a little bit by giving them that space. The problem is that drivers' selfishness, thinking only of themselves, is counterproductive, because their collective selfishness and urge to get ahead without consideration for the traffic around them makes the congestion worse and slows them down along with everyone else. It's been proven that if more drivers would just allow a little more space to form ahead of them, then there'd be far fewer traffic jams. In this, as in most everything else in life, you do yourself more good by being considerate to others.
Also, I'm doing myself good because I'm wasting slightly less gas than I would by constantly starting and stopping. The less you need to brake, the more money you save yourself. So again, grabbing at the short-term perception of personal gain actually leads to personal cost.