A softphone is an interface in your computer. This interface allows you to make and receive calls, just like a regular desk phone or mobile phone. You then connect a wireless headset or regular headphones with a microphone to your softphone so you can talk while you work on the computer.
The reason why so many people prefer to handle their calls via a softphone is because of the simplicity of being able to have everything in the same place. That way, as a user, you do not need to switch between devices while sitting and working at your computer.
Want to know more about what a softphone is and how it works? Then you will find more information here.