Webmasters are always conscious about their page rank. So they use some Pagerank prediction tool to find out their websites future PR.
But there is no need of such a tool to predict page rank. You can manually find out your upcoming PR.
Your PR (pagerank) depends on following factors :-
Number of backlinks to your site,
Frequency of site updation &
Quality of your pages and contents.
If you get inlinks from sites having high page rank, then you will get nice PR. Else your domain should be old enough to get a higher page rank.
Sometimes the pagerank prediction may be wrong. So dont trust any PR predict tools.