>  I am wondering if [it] is necessary [to have these files?] 

No, it isn't strictly necessary to have any particular files on a web server, including any "default" index pages (the ones you listed).

> [W]hat happens if there is no such file name[?]

The server will generally do one of five things:

 * dump a directory list of the files that are available (assuming a default index page does not exist or isn't configured).

 * block access to the requested resource directory (if specified in the server settings - usually via a 403 Forbidden status code).

 * serve a alternate index page (if specified in the server settings).

 * return a 404 status code ("Not Found") (unlikely, but possible). 

 * redirect a user to a valid resource (if specified in the server settings)

> My first guess is that the server would display a complete listing of the site's project directory.

This is the default on a number of web servers, assuming that a directory/default index page combination is requested, and the default index page does not exist.

> Is there any other outcome except a display of the site's full directory?

Yes, see above.
    
> Is there a possible way to avoid this without naming a default page file name?

Yes, configure your server to do one of the things listed above (other than dump a directory listing).

> Does [not having a default index page result] in a loss of SEO ranking?

Maybe. 

 * If you are blocking access to the directory via the server (with a 403 Forbidden) or the server returns a 404, search engines can't crawl your content. Google will definitely not direct people to pages with blocked content (403) or items it thinks don't exist (404).

 * Redirection or serving another file might cause a crawler to index something unrelated to that directory (though this may not be true depending on what you redirect to). This may affect SEO.

 * Dumping a directory may or may not help SEO depending on what is dumped and how Google might rank that content.

I am not sure if Google or any other search engine has a direct penalty for a lack of default index pages, but I can imagine they might interpret it in a manner that might negatively impact SEO.