Using events to trigger emails saves time and money. Effective planning can increase the gains. All you need to do is work out what you want from each intervention.
Some events are common to us all, such as a customer subscribing to an email marketing list. The purpose of an automated response would be to negate the chance of a complaint by using a double opt-in as well as a the first stage in building a relationship. Measurement is the number of unsubscribes, which if quite low would support your methods of gaining subscribers.
Another event is the first purchase. An email containing a coupon would put them in buying mode and you could include a questionnaire. You would obviously check the usage rate of the coupon and the timing of the next purchase.
A most effective trigger, given the use by successful large companies, is the browsing conduct of a subscriber. If they click on a certain page on your site, or better still check the technical information of a particular product, then this should generate an email, perhaps offering a small reduction in price.
An alternative could be a slightly better item if the original was poverty spec. or, of course, vice versa.
If, for instance, you find a low completion rate for emails after a browsing session you should experiment a bit with sending them a little sooner or later after they have viewed the item. It is a difficult call as you want to leave them time to consider the purchase and perhaps buy at the advertised price, but not delay so much that they wander off to another website and buy there.
One that shows you treat your customers as individuals despite using automatic responses is the anniversary. Perhaps a year after subscribing, or maybe their 10th or 100th purchase depending on your product, why not offer a thank-you coupon or entry to a prize draw.
All such interventions should be measured and their effectiveness assessed. Whilst the investment might be small, there is still an investment.
Remember that just because automated responses do not cost much they should not be used without thought.