As the title implies ValueThisWebsite.com is one of the multitude of valuation tools for ballparking the worth of a url. Of course the creators, for whom English is apparently a second language, do not disclose their secret recipe for how they are calculating said value. (Read: take the number with a block of salt.) If you go to another valuation site, you'll see different figures. Nevertheless it aggregates a handful of nifty informative tidbits, including the IP address plotted on a google map, the Alexa ranking, visitation charts, and the reason why I like it for private company research- estimates of daily ad revenues. If you believe its valuation system, then it's useful for comparing public and private companies, e.g. Petsmart.com (public) vs. Petco.com (private). Looking up URLs could give you an idea of the relative size and/or market share of Petco when it isn't spilling its guts to the SEC like the competiton.
One more thing: you might see the 'Websites with Similar Value' tab on the search results page, but it's really not worth your time. It only returns sites with adjacent traffic rankings, not categorical competitors like the aforementioned pet supply stores.