In Vim there are many benefits from having multiple buffers open within the same Vim instance. Buffers share session data of the Vim instance they're in. This has a lot of different benefits, including automatically sharing whatever options or settings have been set (many dynamically or temporarily set), sharing of Vim registers, macros, and command history, all of which are heavily used when editing (if OS handled they would generally share a single clipboard register), Vim-controlled navigation between buffers and windows, Vim-controlled scripting between multiple buffers, Vim-controlled searches of multiple buffers, and more.
Because there are so many benefits of having multiple buffers/files open within the same Vim instance it makes sense to have multiple buffers viewable at once, not to have only one viewable at a time. You lose a lot of editing power if buffers are not open in same Vim instance; it doesn't make sense to open a new file separately in different Vim instance. Moreover, many times split windows are useful to show two different views of the _same_ buffer at once.
Because there are so many benefits of having multiple buffers/files open within the same Vim instance it makes sense to have multiple buffers viewable at once, not to have only one viewable at a time. You lose a lot of editing power if buffers are not open in same Vim instance; it doesn't make sense to open a new file separately in different Vim instance. Moreover, many times split windows are useful to show two different views of the _same_ buffer at once.