Think about how fun it would be if you could spontaneously combust whenever you wanted. BOOM! Ash. Then back to normal. Then BOOM! again. But why on Earth would I want to immolate myself like a Vietnamese monk? Well, there are several practical reasons for which voluntary spontaneous combustion would be very useful.
If I didn't want to do something that involved work, I could just blow up, and wait until someone else did what I was meant to do. Or, if I felt like punching someone, I could blow off my arm, and have it hit them. That way I wouldn't have to bother getting up and walking over to where they were. This would probably also freak them out as well, so I doubt that anyone would ever mess with me again.
Spontaneous combustion would be a great party trick. If someone were to comment on how much they ate, saying, "I feel like I'm going to explode!" I could show off just by doing it. A little reality helps people tone down on hyperbole.
Spontaneous combustion would also be a good source of income. First, I would go on all the talk shows, and demonstrate. This would make me famous. Then, I would go to a bank and ask for money. If they didn't give me any, I could either blow the safe open, or take hostages. If the hostages were annoying, I could blow them up, too.
Spontaneous combustion could also help me from getting hurt. If I were getting shot at, I could blow myself up, and then reform later. Or, if someone threw me off a cliff, I could blow up, and the blast would not only slow my fall, but then it wouldn't really hurt to hit the ground.
Since I've never blown up before, I can't say it wouldn't be without its
problems. Maybe it would sting like a bitch. But maybe if it happened fast enough, I wouldn't feel it at all. On a practical level, I'd have to figure out just how to reform my body, after being blown apart. This would take mind control powers I just don't have yet. But if I work hard enough, eat right and exercise regularly, I'd probably be able to spontaneously combust and reform at will. Then I would rule.