not sure about index file.

[feiticer]

Could someone explain why it's a good idea to have blank index.html or maybe not blank in every folder on the site?

[hunkychop]

so when the user goes to that folder, they see a blank page instead of a directory of all of your files.
ex.
http://johndm.com/images/index.JPG

[tarsem]

Spose u have a folder name health and 50 diff. files in this folder without a file name index.html, now upload this folder to your server and open this u see all the files make a list.

Now rename a file to index.html and again open this from server u see from 50 files only n one file open without any type of listing by default..... You Know this is that file means index.html

[tarsem]

in our school time we see in all books on main page that's name index.... same in website