Why is it better to render video data from my computer into an external hard drive?

Do you mean if you rip a video from a dvd? The only reason I know of is to save space on your internal hard drive. I keep a separate external hd for movies because if you keep the quality of the video, they are huge files and take up too much space.

If you have a rendering of video data on a hard drive, the data of that video need to be read.

The less interruptions on the read, the better the process works.

Every time you write onto a hard disk, the same head that reads the data has to be moved to another part of the hard drives platter, to write the data. Writing data to a hard drive may therefore impact your computers ability to read on a steady high speed from that same drive.