People are mainly concerned with Google. Google use these > http://support.google.com/webmasters/bin/answer.py?hl=en&answer=79812 However Google is not the only thing. Other places use other things to.
There's no importance; "index, follow" is the default treatment (unless otherwise blocked in robots.txt or with X-Robots-Tag), so specifying it is pointless. You only need to add the robots tag if you want to do something other than the default (e.g. follow links, but don't index the page).