Is robots.txt file necessary

I am sorry, i have no idea about it. You can browse to the internet for more details. I used to search only thru internet.

Ventrilo
Website Conversion

If you don't put robots.txt file then there is no any single disadvantage of that. But SEO point of view if you put that file then it is beneficial for you.

Really Robot.txt file is not necessary to the website. It is used to restrict the search engine bots to crowl any parts of the website.
If you not want any restriction to search engine crawling then no use of robot.txt