How does it "command you how to use your computer?" A brew formula is a simple ruby script that lists the packages dependencies, and calls "make; make install". Typically it allows you to set any autoconfig vars and remembers them for next time. This is literally just a thin layer of abstraction over grabbing the tarball and installing the software yourself.

Also brew does not stop you from installing a piece of software literally any other way you prefer if for some reason you don't like what the brew formula is doing, including just creating your own tap.