One of the best seo tools because checking the page rank of internal pages and outgoing links contributes to your own. The main use is comparing scores between multiple pages in the same domain when several match the criteria for being relevant. Checking also warns of low or non-existent page ranks to avoid bleeding PR or discover pages Google might find undesirable.
If this tool is used for scoring a particular set of pages for targets, it's not always the highest rank that counts. An equally qualifying URL would contain key words or the base domain name itself. Keep it Str8!