That is a great question. Why should you step up from Optical to HDMI?
It depends on what you're attaching to make the right decision. There are pros and cons of both, which we will discuss to help you make a good decision to optimize your home theater system.
Both HDMI and Optical pass a digital audio signal from one device to another. So from a soundbar to a TV, or receiver, to a TV and or soundbar. Audio is going back and forth and both cables will do a good job in sending sound. There really is no difference when it comes to passing that kind of multi-channel audio like Dolby Digital that's been around for a long time.
The major difference and why you would step up from Optical to HDMI, is because HDMI cables can pass a higher resolution audio. This includes formats found on Bluray like a Dolby TrueHD or DTS HD Master Audio. If you're looking at an 8K HDMI 2.1, now you're even looking at Dolby Atmos and a much bigger surround sound like a 7.1 or a 9.1. Although HDMI 2.1 is a foolproof method for passing deeper audio, in many cases an HDMI 2.0 with ethernet will work as well. A Premium Certified cable has ethernet, such as any Austere HDMI cable. Where the true fiber optic cable is in place, you'll get that simple Dolby digital audio resolution.
That really is the difference between the two when it comes to audio. If you have a video display there is a big difference between an Optical cable and HDMI. The fiber optic cannot pass the video signal, but HDMI can. If you're looking for one cable to do all the hard work in the system for both video and audio, then HDMI absolutely should be your pick.