Flixmate is required to allow Addoncrop to function as a 4K video downloader because YouTube no longer offers direct download for HD videos with sound. Right now, YouTube only supports download via Dynamic Adaptive Streaming (DASH), which means the video is split into two files - video & audio - which need to be merged to create a watchable video.
When you use our extension as a YouTube HD downloader without Flixmate, the videos you get will come without sound. It requires a special 'FFmeg' tool that merges the video and audio - something that Flixmate has, allowing it to command the extension to merge the audio and video in the background, giving you the high-quality video you were expecting.