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 at 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.

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.