eximconfig option 2: why not send mail directly?

[Griffith Feeney <gfeeney@hawaii.edu>]

I'm in the process of setting up exim/fetchmail/procmail/mutt on Debian
2.2. I've used eximconfig option 2, for which outgoing mail is sent via a
"smarthost" and incoming mail is picked up from one or more POP servers.

Getting incoming mail from POP servers makes sense, since I'm only
intermittantly connected, but why not have exim send mail directly? What is
gained or saved by going through the "smarthost"?

If it makes sense to do so, how to configure exim to send outgoing mail
directly but not accept incoming mail? I've spent several hours going
through the exim docs without finding answers to these questions.