A lot of the more technical details are still beyond me, but the basics is that streaming video files are almost always inferior versions. In general, streaming uploads are optimized to be cost efficient to quickly send to you over the internet. Every video you watch "online" has to be sent to your computer first, even if just in part. It can take multiple hours, depending on the filehost, for an hour of 1080p resolution video to be sent to a computer. But if you fiddle with things like the bitrate, or very simply speaking the amount of "information" that the video has to change each second, you can send the same resolution video so fast that someone can just watch it in real time as it's sent to them. It costs a lot of money to stream videos to people in real time, so anything filehosts can do to reduce how much they have to host and send is helpful to them.
The cost of this is the image quality will be reduced. Blurrier, usually. Sometimes even stuttering if it can't get sent to you fast enough. Downloading the whole file first from a streaming site will prevent it from stopping and starting during playback, but the quality will still be reduced. Sometimes, if the original file was small enough, it doesn't have to be reduced much. For longer ones, it'll probably be worse. Even if the streaming site has a "download option," you're rarely downloading the "original" file that was uploaded but a compressed, optimized version.
For example,
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. The same video, at the same resolution, that was downloaded from 26region's original upload is twice the filesize with over twice the bitrate compared to the file downloaded from rule34video. Depending on the viewer, and their screen, they may not even notice or care, especially if you don't watch videos at fullsize to begin with. But it can get much worse than this. Anything with a lot of movement will cause problems. When watching the actual video, at full size, the difference can be more noticeable than in still frames. You can check for yourself below, and while the stream file screenshot does not at a glance look too much worse than the one from the original file, opening them to full size and flipping between them will make the differences stand out immediately. This is already a blurry frame from the motion blur, but the stream compression makes it that much blurrier on top of it.
In general, f95 prefers to have copies of the original files before any sort of compression/optimization is done on them. Part of it is just the usual "some people always want the best/original version of something," but it is also that the quality can get really bad and having the original version hosted in the OP prevents that from getting spread if that's not what someone wants to download. It's a good standard to have because you can usually find the compressed version on the streaming sites. Better to have the originals and reduce down if needed than be stuck with the inferior versions for everyone.