The from email address from the main newsletters subscription module is used when the module sends out messages from that particular modules. Example:
* Verification Email (if enabled in the module)
* Update Subscription Email (when they click Go)
* Email notifying them that their subscription is updated
Basically all emails that come from the newsletter subscription module use that modules from email address. So you could set this to be subscribe@yoursite.com if you wanted too.
On the Advanced Newsletter module, this is the from email address that is actually used when distributing emails from the newsletter module. So if you wanted to this could be something like newsletters@yoursite.com etc... In many cases the from email address should be the same for both modules but its really up to you and how you want to implement the module.
-Chad