Sometimes you need a quick and easy UI for your application. In the past, these were bare-bones buttons, inputs, and other displays barely usable by even technical users. But the quality has improved for even the quickest-built UIs, and they've become easier to build. Now it's for everyone, from machine learning engineers who need a quick interface to customer support workers who need a wrapper around an internal system.
There's Gradio (acquired by Huggingface, undisclosed amount) for simple UIs for machine learning models. Simple text inputs that are reactive to text outputs, data frame outputs, or even images. Useful for getting a UI to run text-to-text inference (e.g., chatbots, NLP) or text-to-image (e.g., Stable Diffusion). Gradio powers the generic model UIs for Huggingface now.
Then there's Streamlit (acquired by Snowflake for $800mm), which is a similar product but skews more toward the data analyst crowd. It also features reactive components and a primitive but useful layout system.
There's the internal tool category, ranging from Retool (for apps that non-technical internal users use) to Airplane (more focused on UIs built for DevOps and Ops workflows).
Just like NextJS occupies the role of "what's the quickest way to get a full-stack application on the internet?", these tools satisfy the "what's the quickest way to get a UI for my code?". Routine questions now, but as the tech improves and the cost decreases, they become more interesting.