Among other things, it allows websites you're visiting to download plugins necessary to view material on the page without your permission first
Things like shockwave player, midi plugins, and other stuff.
Spyware like Comet Cursor, Cydoor and many others also might get installed without you even noticing it.
And the same unfortunately goes for even nastier material.
This is not to say, that when you uncheck 'install on demand' you won't get spyware on your system.
Any freeware app you download might have some, but at least you retain a certain measure of say about what is or is not being installed on your system.
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