1 To prevent content copying
Some websites disable right click in order to prevent content copying. The webmasters of those sites might want to make sure that no one will steal their content or copy it to their sites.
2 To prevent image downloads
Many image based websites prevent people from right clicking in order to protect their images and prevent people from downloading them or copying them to their websites.
3 To protect intellectual property
Some webmasters disable right clicks in order to protect their intellectual property. This intellectual property can be in the form of text, images or source code.
4 To make the site seem secure
Some institutes like banks might disable right click in order to give the impression to customers that the site is more secure.
5 To prevent form resubmittion
Some sites might disable right click in order to prevent people from reloading the site by mistake and so cause from resubmitting errors.
6 To prevent extraction of direct links
Some webmasters disable right clicking in order to prevent the extraction of direct links.
7 They think it secures the site
Some webmasters might believe that disabling right clicks is a measure that can increase the security of the website. Those webmasters might not know that this security measure can easily be bypassed.