The main difference will be interface. With a DAC playing a thumb drive, you are at the mercy of that DAC's interface (on-screen vs app-based, remote vs no remote etc.). If you have a huge amount of music, and depending on the thumb drive's folder structure and DAC's interface etc., it may be cludgey to navigate.
With a PC, you use the Application of your choice and can control it in a number of ways... physically manipulating the laptop, wireless keyboard with laptop hooked up to a large screen, or even using a mobile app to control the PC app etc. From a PC you can also access the internet and all streaming services.
I have a dedicated PC in my 2-channel setup and another dedicated PC in my HT setup... it's hard to go back once you're spoiled like this. PCs can do... everything :) And you can get good refurbs for super cheap nowadays.