The other thing the toolbar is specifically useful for is that it allows you to easily check backwards, or incoming, links. Directly to the right of the PageRank bar you should notice a "Page Info" button, if you click on this you should see an option for backward links. Selecting that will give you a select of the links Google has in it's public index. You will run into cases where a page has a rank in the PageRank bar but you can find no backward links. This can happen for two reasons. The first is that Google has not spidered the page. Sometimes when Google has not spidered a page the toolbar will guess a ranking based on the domain (assuming it has spidered the domain) this ranking is really just a guess and has no weight at all. This is why pages on free hosts such as Geocities seem to have a high a PageRank, when in reality they've just never been spidered.
The other reason a ranked page may show no backwards links is that Google does not put every page it spiders into it's public index. If a page itself does not have a very high PageRank, perhaps because it only has one inbound link or something of that nature, then while Google will note and count the outward links on that page the page itself will not be listed in the index. This is evident in the various popular directories like DMOZ or Yahoo. If you have a directory listing in an obscure subcategory then that listing will indeed raise your rank but the subcategory itself may not be listed in the index. If you're interested you can do more robust link research with Yahoo's Site Explorer.
Overall PageRank Power is a concept only, it doesn't have any bearing on your actual search rankings, however it is useful in a number of ways. For instance if you were buying a site knowing it's Overall PageRank Power, the total amount of weight it could contribute to an existing site of yours, would be very useful. If you were planning on building a complementary site to one of your current sites, knowing the Overall PageRank Power of the current site would like you know how much benefit you could get by cross-linking them. In that manner you could also use it to decide if you want to make a complementary site. For instance if you know the current #1 site on that topic has a PageRank of "5" and your Overall PageRank Power is a "6" then you can be confident that if you do make this site and cross-promote it with your current site then it will rank well. It is also a good measure because many sites gain more incoming links to their subpages than their homepages so looking at the PageRank of the homepage alone does not give a good measure of exactly how much link weight the site has accumulated.
For more information on using your Overall PageRank Power see the our article entitled "Affiliate Gold."