They are very good for progressive enhancement, for example you could have a web component that wraps a <table> to add fancy features like filtering or drag-and-drop reordering. If JS is disabled or fails to load, the user just gets a plain table, but they still get the content. When this stuff was new, that was much better for the user than what would happen with a front-end framework (they would get a white page), but now server-side rendering is widely available in those frameworks it’s less of a selling point.

They are also good for style encapsulation, i.e. you could drop someone else’s component in your page and not worry about it affecting or being affected by your CSS. Anecdotally I feel like that is less of a common desire now than it was ~10 years ago, with the rise of both “headless” UI libraries (behaviour without dictating appearance) and the prevalence of scoped styles in front-end frameworks.

What does annoy me about the standard is that to use slots you must opt into the shadow DOM, which means that if you’re trying to create reusable components for your own stuff, you can’t style them just by dropping a stylesheet into the page. I’m sure there’s a technical reason why this is the case, but annoying nonetheless.