For reference, WordPress has it's own internal CRON system which is fired off when visitors access your site. This is how scheduled posts are regulated. The same applies to [insert item] if it's on a schedule (i.e. backups, etc).
Normally when you see "doing_wp_cron" in the URL or error log, it's due to the internal CRON being unable to run, or, you've disabled the internal CRON in favor of an external (i.e. crontab -e or through your control panel GUI).
The downside, the internal CRON is fired off for each visitor, which can stress a server on larger sites. Just an FYI, if you're running a large installation with quite a few visitors hitting the site at all hours of the day, an external CRON run may be better. That just from experience working with sites that continually see hundreds of thousands of [unique] visitors a month. On smaller installations it's fine and really won't do much harm, especially if you have your own VPS.