XML was much better than what was there before - which was a different standard for every endpoint and often no structure at all.

XML allowed you to use tools to build automatically. We have other better tools now but back then it was like magic. Download an XSD (the more common option to Relax NG but not superior IMO), point a pre-built tool for it and it'd build strongly typed model classes and validation checkers. Then, when you called the service, chances are it would work first time. It could also be used to write the specification too. That was unheard of before. Often the spec you'd get for service was different to what the endpoint served because keeping documentation up to date was not a priority.

XML then got a little overplayed. For example, XSL transforms allowed you to turn one XML into another and because XHTML existed you have people building entire front ends in it (not recommended). You ended up in a weird hinterland where XML wasn't just for representing structured data but it had built in functionality too. It was not the right tool for that job!

I've not needed it in a long time as I prefer lighter weight formats and I don't miss it.

Just my take, others will have their own!