Edit: I misread the title as "Why should I buy $100 worth of BTC" …I thought the OP was pondering what reasons one would have for possessing Bitcoin. I suppose that still could have been the reason for the question.
It's an excellent question, but I think investment opportunities shouldn't be allowed as answers. That's not to say that there aren't good investment opportunities (including Bitcoin itself), but it's important that Bitcoin be used for actual commerce. It's possible that Bitcoin's monetary properties will make it worth holding regardless of whether it's ever used directly for much commerce. It could prove to be an effective store of value even if merchants never directly accept it on a broad basis. In my opinion that's a compelling reason to hold some Bitcoin, but Bitcoin will be much more compelling if there are plenty of commerce based reasons for holding some of it. I also think you should disallow answers regarding speculation about future commercial uses of Bitcoin. Here are a couple of commerce based reasons for having a few Bitcoins right now:
- to buy drugs on silk road …ok, I had to throw that one out there
- to buy VPN services if you live in a part of the world that has restrictive firewalls for accessing the internet
- an easy way to pay people for services, or settle debts, without being subjected to limited banking hours, or handling paper checks, or waiting days for electronic payments to clear, or paying outrageous fees for international wires
- to save money on a purchase where Bitcoin sales are discounted due to the higher fees/risks that merchants have with other payment options
- to buy goods and services from eCommerce sites if you happen to live in an area where other payment options are cost prohibitive or outright impossible (there are many countries from which eCommerce websites either cannot accept credit card payments, or the fees are ridiculously high…like 20% high)
- to make purchases without divulging bank account or credit card information that could be stolen from a merchant with poor security practices
There are a lot of things that can be bought with Bitcoin, but not necessarily exclusively with Bitcoin or where Bitcoin offers any particular advantage over other payment methods. I also think that the unique properties and capabilities of Bitcoin transactions will make it a compelling tool for many types of transactions in the future, but that's speculation about the future commercial uses of Bitcoin. I also didn't list illegal uses of Bitcoin (aside from silk road) because if the predominant use of Bitcoin is for illegal activity, then Bitcoin will never see widespread use outside of criminal organizations.
What else can be added to this list that isn't speculation about the future utility of Bitcoin, that isn't investment oriented and that isn't illegal in nature? The longer this list is, the better off Bitcoin will be.