Reasons why you would want to change the default document can be for many reasons here are some examples to why you may want to change your default document.
Example 1: ASP.NET websites use default.aspx, you will want to make sure default.aspx is on the top of the list
Example 2: PHP websites use index.php, you will want to make sure index.php is on the top of the list
Example 3: Website is down for maintenance, You may want to make sure your website resolves to maintenance.html
How to change the default document (index.php, default.aspx, index.html, etc)
Step 1: Log into Plesk
Step 2: Click on "Websites" Tab
Step 3: Select "Hosting setting"
Step 4: Scroll down "Hosting Settings" Make the changes if any as necessary , then click "OK"
Step 5: Click on "Website Server Setting"
Step 6: Under "Default documents" modify as necessary
Step 7: Click "OK" and [DONE]