Imagine this scenario:

You're using Webform to create a "Contact Us" form at www.exampleclient.com. Your client wants an email copy of all form submissions, but you're under a really tight deadline, so you just go with the default settings for the "from" address. The default "from" name and email address are the site name and email address from the Site Information page (/admin/settings/site-information).

Whew! Done!

But then... a few days go by... and as your client's website begins to see some heavy use, you see your personal inbox filling up with email replies to contact requests by your client. That's odd, you think. I never filled out the contact form. Why am I getting replies to contact requests I never made? You open the emails, and realize that your client is just hitting "reply" on the email notification that they get... and that reply is going straight to the default email address from the Site Information page... which is webmaster@exampleclient.com... which is... you.

Oops!