The two iOS widgets that I find the most usable and also provide quite a lot of utility are the MyRadar (weather) and Kasa (smart home) widgets.

I can get most weather information I need from the MyRadar widget; rarely do I need to open the app. If I do, it’s mostly to check weather in other areas like my parent’s home during hurricane season.

Likewise with the Kasa widget. The only time I open the app these days is to setup a new device or an automation/scene.

Useful widgets exist, but I imagine their use cases are fairly limited to mostly trivial things. For more complex applications, it probably simply isn’t worth the dev time to create and maintain a widget.