For a long time, I didn't compress my movies after ripping them because I wasn't worried about storage space and wanted to keep the quality of the movie as "pure" as possible when compared to that of the Blu-ray disc.
I had file sizes that were over 40 GB just for a single movie. That equates to a bit rate of 35 Mb/sec for a two and half hour movie and ended up presenting a few problems with Plex and Kodi.
Because of the enormous file size for one movie, my Plex server couldn't handle very many simultaneous streams. More often than not, Plex transcodes content (converts it on the fly) to lower the bit rate to something more reasonable than 35Mb/sec and to make it compatible with whatever device is streaming said content. Let's use HD audio as an example since most of my content only contains an HD audio track. Most of the devices that run Plex don't understand the HD audio formats. So Plex transcodes the audio to a format that the streaming device can understand. This requires a lot of processing power.
In addition to audio, Plex almost always transcodes the video portion of content as well. Again, this requires a lot of processing power. With Plex having to transcode such large files, my processor was simply being overworked.
Kodi's problem was not nearly as severe, but was still annoying. Because my movie files were so big and had such a high bit rate, seeking didn't work very well. Kodi can seek forwards or backwards up to 16X normal speed. That doesn't end up working very well with a file that has a bit rate of 35 Mb/sec.
What I eventually discovered is that I could compress the video portion of the movie files without any noticeable difference in quality while leaving the audio untouched. The smaller file sizes means Plex doesn't have to work as hard and Kodi can seek through a movie without any hiccups. Yes, Plex still has to transcode the HD audio and yes, I could add other audio formats to my movies, but that would make the file bigger and in the name of conserving at least a little bit of storage space, my files only contain an HD audio track. After all, the visual and audio quality of Blu-ray is why we buy movies on Blu-ray instead streaming them, right?