if you have a good preamp and soundcard you dont NEED another AD/DA converter, as your soundcard, I assume, had AD/DA converters.....
one might look for an external converter because they are looking for higher quality conversions, in the same way one might step up from a Shure SM57 to a NeumanU87 for vocals.....they are both microphones and do basically the same thing.....one just seems better tailored to do it than the other.....
you can take and clip recorded with your soundcard and a clip recorded through a Lucid converter (from exact sources) and play it back through good monitors.....the difference amy surprise you....it wont knock you out of your chair, but the clarity and subtle differences are there....
for someone like me, right now the difference doesnt justify the expense...the Delta converters do a good enough job for what im doing....whether or not you NEED them depends on what you are willing to spend and what quality of results you are after.....