Yes, there is a reason. For example, if you have your page organized in certain folder and loaded in the server, then when you inquire the server like this:
will do one thing: It will go to that location and since you didn’t explicitly specify which page to load(what file is your main page), all servers will attempt to open an _index.htm_l file. Of course, this is more for convenience than anything else. Also this is the standard. Since most people never type the address of a site, I think it is ok to use index.html as your main cover page. If you don’t create an index.html page, then no big deal. You can always load your page like this:
and then, instead of having an index.html file in your root folder, you need to have a file called my-main-page.html file.
I hope this helps,