One reason is so that invocations of ssh outside of the context of user invocation of ssh at the command line will also have these customizations included. This is especially important for ssh, which has emerged as a main security interoperability tool for Unix systems.
For example, if you use rsync, the tunneling and host alias conventions you set up in .ssh/ will carry over transparently to the ssh tunnel used by rsync.
Another example would be invocations of ssh in scripts (sh/bash scripts, even) that will not or might not read your .zshrc.