Why it is advised to keep the first page name as index.php / index.html???

Answer 1

It's because when you use internet browser and go to some page www.example.com, browser automatically loads index page (unless it's specified differently in the configuration).

www.example.com is in the default case www.example.com/index.html (or php)

Maybe you noticed that in a page URL when you go to that page there is not index.html/php, it still loads index.

*short answer: its easier, but if you really don't like it, just add redirections to other htmls in index.html

Answer 2

If it's called homepage.html, the page must be www.mywebsite.com/homepage.html for the page to load.