Yes. Effects like noise reduction take a looooooong time. Once rendered, the clip plays fine. If you delete the render files, it will take a looooooong time to regenerate the render files and you won't be able to play the clip properly until the rendering completes. (The clip will play unless you have the "If a frame drops, stop playback and warn set in the Preferences pane. But it will likely play sluggishly in fits and starts, skipping a lot of video frames to keep up with the audio.) Note I'm still talking about CPU/GPU intensive rendering, like that with noise reduction.
Back to "normal" cases:
You can still edit and play the project, but in some cases things might get slow. You can enable the "video frames were dropped" warning in the preferences pane and other similar warnings so you know when it happens. Oddly, sometimes, even though all the rendering is done, playback (seems to happen only on certain clips, and not as often lately) for me will just stop. It could take several tries for it to work right. Sometimes things seem to start out sluggish when starting playback.
It stores them to improve performance. Maybe your system is at times bogged down with other activity, slowing down the rendering, meaning that video frames may be skipped.
Yes, you're wasting your time to delete them again and again unless you have a lot of "leftover" render files from previous effects settings, in which case deleting "unused" render files will save you some disk space. You can use Cmd-Ctrl-J (Library Properties) to see how much space the render files are taking up.