Who cares why do you care about PR when you need to concentrate on the important things Building Backlinks / Rankings / Traffic
It's important to note that this is just talking about toolbar PageRank. The real PageRank that Google uses is constantly being updated. By the time it reaches the toolbar, it's months old.
The only time I think pr can be handy is when your looking at your competitors backlinks, and you can display then in order of pagerank, good to get yourself some backlinks by doing the same things if you can. Other than that its pretty pointless.
forget about PR It is a static measurement of your site that only had an influence on Google at the time of the update Googles Webmaster tools show all your backlinks and is up to date. IGNORE PR TOOL BAR