why is it important to update websites?

Answer 1

Ditto the above plus, if you *expect* to have return visitors, you gotta give 'em a reason to return... If they don't see something new when they come back, then, "yawn" why bother? Increase traffic, especially if you're running something to garner yourself some $$$ and spread your name around

Answer 2

One of the criteria in ranking search engines is the fact that they do get updated. Though some old static sites can rank high, they do get penalised for not having fresh content.