Back in the olden days, when macintoshes were new, it took literal minutes to load the program off the disk. This was an improvement upon programs taking hours to load off cassette tape. So when you closed the last document, you didn't want to have to wait to load the program again if you were just going to start a new document in the same program. It just kind of persisted from there.

It's fine to keep them active in the background, but the annoying thing is that Mac still shows them when tabbing, even though I closed them on purpose. I closed them because I do not want them to show up on my workspace.