In marketing: establishing trust is critical if you want to convert strangers into customers and raving fans. So if you pitch a sub-par product as the greatest invention ever created and people are continuously disappointed before the product catches up to the hype, you’re in trouble.
In relationships: no one wants to feel like they need to sign paperwork (e.g., FrieNDA) before sharing something with you. Using discretion with respect to what’s public information and what’s to be kept secret will save everyone involved a world of pain and embarrassment.
In web applications: if a user signs up for your application using a new (or rarely used) email address and there’s an influx of spam to that email address in the days that follow, they’ll probably deduce (correctly) that their information has been sold.
There are of course less malicious mistakes that can be made in web applications, but the ramifications are no less dire.