24/0, 20/4, 18/6 etc will all keep a plant veging, allowing you to grow the plants as big as possible by giving them more time to grow. This can mean a higher yield at the cost of more time and electricity costs. This means you can grow less plants and still get a high yield off larger plants. It's been argued back and forth whether 24/0 light or some cycle with a dark period (18/6) is better, but there really hasn't been solid proof to prove either side. I think a dark period is good, since everything needs rest once in a while, but again there has been no solid proof either way.
Growing 12/12 from seedling forces the plant to start producing flowers as early as possible, leading to a smaller plant and generally lower yield as it has had no time to reach it's potential in the veg stage, but this leads to a quicker harvest. This means you can get a faster harvest, but you will need to grow more plants to get the same yield. I imagine this would be great in a SOG setup, or if you don't have a separate area for veging plants