You do need something that can at least accept emails from localhost (such as cron emails) and either put them in a local mailbox or forward them to an upstream smtp server. Sendmail provided in base will do this, although OpenSMTPD is probably easier to configure.
If your ISP stores your email in a mailbox, which you access via POP3/IMAP/etc, you don't need an SMTP daemon for receiving email. If you have email delivered to you via SMTP (which is unlikely if you just have a workstation) you need something to receive those emails and put them in a mailbox. Again Sendmail can do this in base although it's perfectly reasonable to replace Sendmail with something else. I usually use Postfix, but that's probably overkill compared to OpenSMTPD if you just need something to handle local emails and submit them to your ISP.
