Intelliupdate is special algorithm that computes time for every feed's update. You can see results from this algorithm then checking last update and next update times and calculating values in different feeds.
Intelliupdate wants to be intelligent. Also it wants to do no harm to webservers serving RSS feeds. Novadays there are so many users of RSS and it's not good idea to make little update period for every feed. Also - take Slashdot example - if you are polling Slashdot once in 10 minutes then you will soon get banned.
Intelliupdate tries to do it. For example if there is only one new article in few days in some feed, there is no point of polling this feed every 10 minutes. And for example if there are new items every 10 minutes then Feedreader polls for new items every 10 minutes.
That's why Intelliupdate is default for every feed. For normal person and normal feed it should be enough. If some feed behaves diferently then user can change this option.
Let's get my own example I have 40 feeds what I read. I have only changed this period in three feeds. In all other feeds this Intelliupdate works well..
And you can always press Control+Shitf+F5 to do manual update of all feeds.