Very important to drop this too. We need to understnad  that the value of something is not the same as its price; value is what people perceive a product is worth, while price is what they pay for it. Even so, value and price go hand in hand: the price of something is directly related to its value and vice versa.

The market value of Bitcoin; that is, the money that people are willing to pay for it, follows the same old basic demand and supply rule: a high demand increases its price and a low demand decreases it.

According to an article in the Economist, the increasing trend in the price of Bitcoin is what drives people to invest in it.
People are investing because they believe that, following the trend so far, they would be able to sell their Bitcoins for a much higher price in the future, which the article argues is a perfect example of the greater-fool theory.

You can read more in the  Economist article.