In certain situations, using 3CX on your computer as a "soft phone" makes a lot of sense. Mostly this would be "call center" or "receptionist" types that are on the phone a lot, and also somewhat tethered to a desk. This is unrelated to using 3CX on a smart phone for use on-the-go (though that's an option, too - see this article).
With a soft phone, you pair a nice wireless headset with your computer, a second monitor for watching extension status (helpful in receptionist scenarios), and get high quality sound and save strain from using a wired handset, while allowing much wider freedom of movement during the call.
Note that soft phones, smart phone apps, and traditional desk phones can be used interchangeably, all on the same extension. That way you can switch back and forth as you like.
To get started with the soft phone, just request a "Welcome Email" for your extension from Pinnacle. You may have already received this when you first set up your extension or smart phone app. There is a link there that you can use to log into the phone system on your extension, see the "switchboard" (everyone's in/out/on the phone status), as well as a browser extension to allow you to click and call right from a web page (often a CRM).