I've started to do fartleks over the last couple of months. It's purely because I want to fight against postman's syndrome.
Postman's syndrome is named after the phenomenon of our bodies getting used to the same activity, no matter what it is. It's named after the postman, as they do a TON of walking but aren't always the trimmest of people. Their bodies, like ours, get used to the same activity day after day. Our bodies don't have to work as hard when we're repeating the same activity, and because of that, we don't burn fat as well.
It requires us to do different things. First, if we're only running, we need to change our pace through fartleks or intervals or hills. Even better, if we can cross train by swimming, biking or doing something else, our bodies will try to work harder by doing different things.
In a recent study done over a 15-week period, people who did interval running for 20 minutes burned three times the fat compared to people who ran steadily for 40 minutes. That's not just bodily efficiency--it's also time efficiency.
I know that my body's burning fat because my heart always beats faster and I sweat more when I'm doing fartleks. They don't have to be organized (which fits me) and they're not always fun, but I know it's helped me.