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).
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 firstname.lastname@example.org... which is... you.
You didn't have time earlier since you were under a tight deadline. That's okay. Now, you go back and play with the webform settings. You change the "from" name and address. Instead of the site defaults, you choose the form components corresponding to the name and address provided by your client's customers. Great! Now, instead of being from email@example.com, the emails will be from firstname.lastname@example.org and email@example.com.
A couple weeks go by. Your phone rings. It's your client, and he's angry. You changed something on my site a couple weeks ago!! It was fine before, but now every contact submission notice is going into my spam folder!!! Do you have any idea how many customers I've lost!?
Why did this happen? It's because many email providers, like Gmail, count it against you when the email sender isn't the same as the email's "from" header. It's a great way to filter out spammers (or worse, phishers), since they usually use their spam servers to originate emails from, say, your bank.
So, without realizing it, you've just made Gmail think you're a spammer. Your server has been sending emails from firstname.lastname@example.org, but of course, www.exampleclient.com isn't supposed to be sending emails from email@example.com! So, Gmail marks it as spam, and your client loses a ton of business.
The solution is to use the "Reply-To" email header. Webform doesn't let you set a reply-to address, but the Webform Reply To module does.
Using this module, you can set a separate Reply-To address for your outgoing emails in Webform. The "from" name and address should be something like "Contact Request ", while the Reply-To is set to the form component corresponding to your client's customer's email address.
Then, when your client hits "reply" on the email, it will already be pre-filled with that client's customer's email! No more spam, no more lost emails!