It is always a good idea to know your competition and one aspect is to know which other websites and domains they own to get an idea of their capability to push a website. This can be done by checking other websites that are hosted at the same IP of a website that you want to check out. This does not always bring the desired result though and is only useful if the competitor uses his own server. Additional IPs and servers make it more difficulty but it is still a good way to learn a few things about the competitor.