For this purpose, it succeeded and did a much better job at it than alternatives. HDMI still makes far more sense for use in a home theater environment than DisplayPort thanks to features like ARC.
While HDMI is nice for consumers because it carries audio/data, SDI cables are cheap (just a single coax cable!) and easy to route (T-splitters are a thing!).
SDI does not support HDCP, however.
I'm using my LG TV as monitor for a PC and forced to use HDMI.
Model name is AORUS FO48U.
My current TV will only put out an inferior "headphone" mix over the 3.5mm connection, and the SPDIF connection is co-axial on the tv, but optical on the speaker. Having to run a powered converter box just to get audio from my tv to a speaker feels like such a step backwards.
It sounds like you just got bad luck with your TV having a bad 3.5mm jack and not supporting toslink, as most I've seen will at the very least support the latter, and often have a separate "line out" port for the former.
I know 3.5 is worse on a technically, but I've never been able to actually notice the difference.
SPDIF (both optical and coaxial) support sending multiple audio channels in their original encoding, to support surround sound. The receiving device needs to support decoding of the audio, which is why there is often an option to force the SPDIF output to use PCM stereo instead of surround sound.
If you have a cheap TV and a good hifi, you'll want to use SPDIF or HDMI so that your audio isn't ruined by poor quality of the audio chipset in the TV.
But the difference is definitely noticeably, even to my relatively forgiving ear.
(Also, unique for optical connections, it's easier to avoid ground loop hums.)