Because with a dedicated application you get a dedicated window that is easily findable because it has another icon as the browser you use. I usually have Firefox open with more than 20 Tabs open (TreeStyleTabs). Finding the Ext Documentation is then at least two klicks: Switch to Browser + Find Tab and klick on Tab.
I find it better to have a dedicated window for this. (But that would also be achievable with Chrome + Application Link) but that does not save tha scroll-position of tabs as the ExtDocSuite does. (WebKit limitation).
Additionally, I found the current 3.2.0 supplied documentation extremely lacking. Some classes where missing, at the top where additional links from the old version of the documentation browser, etc. etc.
ExtDocSuite fixes this shortcomings, and also adds all of the ux/air documentation to the mix, which I find quite handy. Additionally since 3.2.0 you get in page search which is a little bit more intelligent than using the browsers CTRL+F... So if you hadn't tried out ExtDocSuite, give it a try, its IMHO much better then what the ExtJS Team provided, perhaps you'll like it.