Below are a few reasons why one would use the robots.txt file.
1. Not all robots which visit your website have good intentions! There are many, many robots out there whose sole purpose is to scan your website and extract your email address for spamming purposes! A list of the "evil" ones later.
2. You may not be finished building your website (under construction) or sections may be date/ sensitive. I for example excluded all robots from any page of my website whilst I was designing it. I did not want a half complete un-optimized page with an incomplete link structure to be indexed, as if found, it would reflect badly on myself and ABAKUS. I only let the robots in when the site was ready. This is not only useful for new websites being built but also for old ones getting re-launched.
3. You may well have a membership area that you do not wish to be visible in googles cache. Not letting the robot in is one way to stop this.
4. There are certain things you may wish to keep private. If you have a look at the abakus robots.txt file (http://www.abakus-internet-marketing.de/robots.txt) You will notice I use it to stop indexation of unnecessary forum files/profiles for privacy reasons. Some webmasters also block robots from their cgi-bin or image directories.