Alright, here's the thing about natural selection: The fish isn't going to develop limbs and a broader body if environmental factors favor the slender, streamlined body type. If a fish evolves to the point where it has developed primitive limbs, then environmental factors must be such that these limbs are either beneficial or make no difference. Limbs are going to make a pretty big difference, so they must have some benefit. So what factors would favor limbs? It's likely that the fish population had to spend brief amounts of time on wet land or in shallow water. This could be to get between two very close bodies of water that had been separated by drought or some other geographical factor. It could also be that there was a certain predator that could not reach them in those areas, so the fish would wait in shallow or no water while the creature moved away. These fish now need to be able to stay out of the water for certain periods of time without suffocating or dehydrating, and they would need to be able to propel themselves in little to no water. In this case, the squat, flat underbelly body type would have been better, as they could have remain upright while the streamlined fish would fall on their sides. Stronger fins and tails would also have been favored, because the fish would have needed to drag themselves in and out of the water, and their simple back and forth motion wouldn't have been enough. So, the limbs would have been an advantage to this particular population.