It has a learning curve like all programs have and easy to follow “how to get started with RawTherapee” -type documentation and videos are scarce at best.
I edit non-raw images also with RT, for example jpegs taken with mobile phone. I start with 13 Megapixel more or less miserable looking photo and end up with reasonably sized (fullhd size, etc.) pleasing photo. It has very useful and easy to use (once you get a hang on it) tools like the already mentioned rotation, cropping and perspective correction, but also exposure editing, shadow and highlight control, addition of local contrast “pop”, various tools to alter colors etc.
One tool I especially like is the post resize sharpening, especially RL deconvolution. Mobile phone jpegs often lack sharpness and after down scaling situation is even worse. This tool applies sharpening to photos after resizing and usually leads to much more pleasing final jpeg.
You can make a profile with settings you like and apply it to multiple images, edit photos roughly in File Browser very quickly and put multiple images to batch queue.