Why?

To slow down spam transmission, hitting spammers where it counts. Fewer spams sent -> fewer dollars received.
To make open relays fill with spam, hopefully waking up the sleeping administrators.
To cause issues at open proxies, hopefully waking up THOSE sleeping administrators.
To take additional actions at SMTP time in order to handle spam as far upstream as possible.

To slow down spam transmission, hitting spammers where it counts. Fewer spams sent -> fewer dollars received.