Most desktops (Windows) have a mic and earphone jack so you would not need the adapter wire. If it has only one 3.5 jack then you use the adapter.
Same thing on windows laptops actually. I have both jack layouts at home on different laptops.
If you are asking because it doesn't work, Song, certain computers used a different wiring scheme to connect the jacks and plugs which is a problem. (For example Mac Pro) Search headset adapter for your particular computer model and find the appropriate one if available in your area. Don't let anyone tell you they are all the same.
