Donnie, It's complicated, and I'm not the best person to explain it. Here's a lot of information from PS Audio themselves on each of the two components of the combo:
http://www.psaudio.com/ps/products/description/perfectwave-transport?cat=http://www.psaudio.com/ps/products/description/perfectwave-dac?cat=Bottom line: I don't know there's probably not much that it does that your laptop "combo" doesn't. I have ZERO desire in having a laptop in my musical system. Laptops are for something else for me. I know they're perfect for many, but I don't dig using them in a music system. I love my cds, I have tons of them (possibly even LITERALLY) and I wanted to buy something that really made them shine and that I could also incorporate my video system into. This combo seems to do that very well, at least so far.
The thing that this combo does that your set up doesn't is allow the use of an I2S connection. This apparently means that the data doesn't have to undergo a splitting and recombining. It seems to me that this makes a difference. For Redbook I've tried coupling the transport via coaxial, glass toslilnk, and I2S via HDMI, and the latter clearly sounds a bit better, more 'ease" and naturalness to the sound. Also I don't know if you can do this with your system, but the "Native" feature means that this can be a Non-oversampling DAC, or there are five over-sampling choices to use. I really so far think the non-oversampling is possibly my favorite in all three sources I'm using. There are also five different digital filters to choose from, including apodizing filters that are the latest big deals, used in the top dollar Meridian processors that seem to be a big rage. There doesn't really seem to be a huge difference so far between these but it's fun to play around with.
There's the RAM that stores a few minutes of the disc (very quickly) and helps it to become an error-free version of the disc the way EAC etc. does in your laptop. And there's a Digital Lens built into the transport as well. I remember Randy the forum demigod couldn't do without his Digital Lens for the longest time, it's a very interesting component.
So far it seems a very promising pair of components. There's a lot of breakin that has to go on between the compnents and the cabling that came with it (excellent cables!) but so far it seems to be a great sounding and versatile combo that's fun to play with and suits my needs admirably. I have to say that it made me realize one thing categorically: my Sony SCD-XA5400ES and the ZCAC-1 are VERY GOOD, GIANT-KILLING components. This combo is clearly better, but it's not an earth-shattering difference, not right now at least and in my experience most components don't change radically in break in. But I'm happy so far. The fit and finish, which I really appreciate more and more as time goes by, is excellent. There's a flexibility that seems to mean I can tailor the sound a bit more consistently than tube-rolling and so far many different types of recordings sound really good. I spent a lot of money, but I wanted to get something that will make me feel really proud and which I can use for many purposes and have fun with. So far I think I selected well.
So read the pages please, I think they'll answer your question.