Say you want a wordpress blog on the same system as drupal and drupal is installed on the root; can you do it? Or what about static pages existing within the part of the site which is powered by drupal; for things like SVG?
Drupal .access blocks viewing of drupal directories and takes over the delivery of content that does not actually exist. place a static file in the root of a drupal installation and you can access it without running any drupal code. Create a directory on the server and access it and it works as if drupal were not installed.
WordPress also takes over delivering pages for files that do not actually exist. One of the details of .access is that apache first uses .access from the root adds to it (or over-rights it) in each sub directory up to the content. So for http://www.example.com/foo/bar/ Apache would look for and read if they exist at least three .access files: the httpdocs directory of the site, foo directory, and bar directory.
For WordPress installed in http://www.example.com/blog/ with drupal installed in the root wordpress’s .access would take over the handling of files in the blog directory usings its .access file. This access file would be installed the same way regardless of drupal. Normally wordpress can be installed with a few clicks in the control panel of the site. Optionally it can be installed old school by uploading, it creating a database, and running the install.php.
Drupal can run with other applications, the .access in the new application’s directory setup the server the way the application needs apache to be.