Opening up from 2.8 to 1.4 gives you 2 stops (4 times) more light.
In the context of low light, if your shutter speed is 1/30s at f/2.8, you can shoot at 1/125s at f/1.4. Helpful for shaky hands or moving subjects.
Or if you're worried about high ISO noise, it allows you to drop from ISO800 to ISO200 if you keep the shutter speed constant.
[Edit]
Note that your DOF (Depth of Field) is MUCH narrower at f/1.4, which may cause focusing issues.
Hope it helps.