Because that's the reason why you are using bitcoins. I could tell you that bitcoins are great for international money transfers or that it is a technological/social project with many interesting cryptography concepts, but why would you or your parents care?
You do not need to make international transfers, you certainly are not studying any of the underlying technology and you should not be making high risk investments if you are young and you do not have your own money. That's why your parents cut your allowance and they did great.
You should only be spending money you don't need with bitcoins. Money you don't need is something that you don't have, because you do not work to earn it.
You listen to me, kid.
Get your allowance back, beg if you have to.
Then you take that $10 a week and you save it.
And you save it and save it and save it.
Get yourself a nice wad of cash going, you need to be responsible with this money, because you have a goal.
Keep saving that fucking money like you're a pro, don't get weak.
Save up until you've got $100-150, get it in $5 so it feels nice and fat in your pocket. Feel that wad of cash? That's success.
Now you take that fat wad of cash to school or the park or wherever teens hang out. Say you're looking for some nug/bud/weed/pot.
Now, don't be stupid, have some friends with you, you're not going to lose this shit now, you've earned it.
Now you ask around until you've got the hook up, and drop all $150 on a fat sack of the greenest stinkiest shit you've ever imagined.
Smoke it. Smoke that shit. Smoke as much as you can, go wild. Hell, invite some underage chicks over, get them stoned too, they'll get REAL excited.
And once you're fucking blazing up with some hot chicks or at least a nice full bottle of lube, and you're just going at that shit, you ask yourself:
Is this worse than Bitcoin?
One word: Satori.