Because plugins usually open split buffers that are horizontal by default.

There's a bunch of different commands in vim that open new split windows. :help, :diffsplit, C-], and so on. I'd rather specify my split policy once rather than remember to type :vert or C-w v everytime. (I only found out about C-w v and C-w s after reading OP, so that exacerbated the problem.)

Imagine new plugins that split windows to open tree explorers or previews or whatnot. I'd like splits they create to automatically adopt my policy.