Because I enjoy running and I'm training for Tough Mudder, which is a half marathon. I spent last month primarily running so I could get my endurance up and to get faster. I also did it so I could trim down before I started a strength training program. And the statement you wrote that I bolded is wildly inaccurate.
"Let's start with the muscle-building scenario. 'Doing cardio first will induce fatigue that may compromise technique and possibly increase risk of injury,' explains Fabio Comana, director of Continuing Education for the National Academy of Sports Medicine. Exhausting oneself with a big run right before weights and resistance training doesn't just up the risk of injury, it also means you'll have less energy to throw into a really good weight training session."
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If you simply want to "lose weight" then yes, doing lots of cardio, whether it be before or after strength training, is good and the order is irrelevant. But if you're like me and you want to put on a couple pounds of muscle tissue before trying to lean out, the order does matter. A gym just opened up near my house and I signed up because I wanted to start working with free weights and heavy lifting. My current goal is being able to squat my own bodyweight and I won't be able to achieve that very well and work up to that quickly if I run 2-3 miles before my lifting session.