1. Render files are necessary if you want to watch your edit in real time on your timeline, as they are the full versions of the video with any effects or transitions you may have added. Though the Final Cut timeline is only "editing" a proxy of your video, the render files are real video files generated by FCP so that you can watch the final product without exporting.
2. No, because you still need them. At least, FCP assumes that you do.
3. Your project lives in a variety of places while editing, and you can't delete any of it unless you are fine not watching your full video with effects and transitions on your timeline. In other words, if you delete the render files, all of those effect points are going to be red and un-rendered. You can, if you really need the space, leave them un-rendered and export your full video without problems, as FCP will only render for export and not create separate render files.
4. Putting something into the trash doesn't delete it, but marks it for deletion. It's still there.

Feel free to delete your render files once you have exported your project and are completely done with it. But only then!