Signal distribution (specifically audio and video) uses it heavily with a smattering of different protocols building on RTP. Major examples are things like media production facilities running on SMPTE 2110, live and installed sound systems built around Dante/AES67, IPTV systems in hotels, and network media systems within office environments.

Precision Time Protocol (which many if the above are also dependant on) is also based around it and used across a number of domains where tight clock sync is required.

Use it all the time. If you watched the fireworks on TV in the U.K. last night that was sent via multicast from the river for example.

Closed circuit IPTV systems use it constantly. I’ve got about 500 addresses ok my network encoding different video and audio channels for desktop use.

That said I only use addresses in the 239/8 and 224/8.