Generally I use the GUI for most things. When I go to the CMD line it for these reasons :

1) I either can't find the option / CMD that I need within the "helpful" Windows XP GUI
2) It is faster to use the CMD Line eg. ipconfig /all rather than through network properties
3) So that I dont get hassled by a Microsoft paperclip or other "useful" Microsoft "features" when performing a simple request
4) For things that i have never seen in the GUI (or can't be bothered looking for) eg tracert

In short. If it is quicker or easier the CMD line is the way to go