Yes robot.txt is necessary. It is used to allow or disallow crawlers from indexing pages.
I agree that there is usually at least something worth placing in the file.
The point I was trying to make is that even if there is nothing at all to go in the file that it is still worth having the file there just to avois all the error messages in the log.
I also seem to remember a few years ago that there was a search engine that wouldn’t search your site at all unless it at least was able to find the robots.txt file.
It’s not necessary to use Robots.txt file. It’s just used to prevent crawlers from indexing any of your personal files, pages etc.
Thanks for the reply 
everyday is a school day!
When a search engine first enters your site the first thing it does is requests robots.txt. If that file isn’t there your site returns a 404 and logs it. That’s what he meant by it reducing the number of 404 errors in your logs.
You can have or ignore having robots.txt on your websites but as its an accepted standard for all search engines, its better that you have it on your website and in this case you can have more control of pages that search engines are going to crawl from your website. I suggest to have it on your website.
I did not know this either! Can you explain how this works? I mean by having the file there how does it stop all the error logs,
Sorry if it sounds a silly question, but i am new to SEO so trying to learn as much as I can
I think it’s more useful to actually do something with the robots.txt file than just have it as a stop gap for the 404 errors. Robots.txt (as used by Google) actually allows for a directive called sitemap: which allows you to specify the location of your websites XML sitemap (thereby helping search engines better index your content). As such it might pay off to include that within the robots.txt file to help spiders find out what’s available for indexing on your site.
PS: The sitemap extension is supported by Google, Bing, Yahoo, Ask! and others, so it’s widely implemented.