The most basic reason to use the internet to collect data is because generally you can collect more data for less money. You are limiting the representativeness of your sample when you do this, however. There are firms that suggest that they can provide representative population samples over the internet, but those claims would have to be assessed on a study by study basis.
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Presumably you collect data to show some correlation or limits. If you don't check the data you use is valid then any result you get from processing it is suspect.