It wasn't very useful to start with and that took it to $20,000.

1. BTC is useful as a store of wealth. A rich saudi would be very happy to dump their fortune into BTC and then wait several years. It would be much safer than storing it in banks where they are held hostage in hotels until they agree to pay N% of their fortune to be released. (Apparently this actually happens.)

2. BTC is a speculator's currency. The whole point is to speculate, and not much more than that. But that's literally the point, and the fact that it's absurd doesn't change how real it is.

3. Fluctuations will be with BTC forever due to #2. This has always been true and will continue to be true. Periods of inactivity will become longer, but then eventually speculators will notice it's a lucrative deal again.

4. Speculators have a crucial ability: they can manipulate the market. It's common for people to dump N million into the market to cause a small rally, at which point everyone else starts buying. This happened to Ripple. No one cared about Ripple until however many months ago that was, when it suddenly surged overnight due to this.

5. BTC has immense inertia because it was the first, and continues to be the most valuable. As long as the core team doesn't screw with it at all – including trying to "extend" it with bigger block sizes or any other convenience features – then BTC should stay a fine store of wealth. Satoshi planned for the long term.

6, The above points do not hold for other currencies. Others have some of these points, but not all of them. Hence BTC remains a solid bet.

7. All of this can be mistaken, but after seeing this behavior many, many times, at this point it's fine to gamble that you won't be wrong this time.

8. Don't put in more money than you're willing to lose.