Two fold.

1st it keeps the spam down. Under the old system anyone could harvest email addresses and then send spam.

Under this system if you send spam Ebay knows who you are and can terminate your account.

It is also designed to discouraged Non-auction transcations which Ebay doesn't make money off of.

Ebay is not reading your emails, their are too many for that. But this system guarantees that if contact another person for an illegimate purpose and that person complains to ebay, Ebay can connect the sender to the email.

Remember also that this only realyy affects the initial contact. Everytime I have asked a question under the new system I have included my email address. The response I got was an email to direct to me.