Why would I invest in bitcoin? All sorts of reasons ranging from pure curiosity to love of new technology to investment diversification. Bitcoin is used as currency to buy things and as a store of value, i.e., a place to house your savings with hopes of value appreciation or hedging against the depreciation of stocks or savings held in fiat currency. One example often used is that of Cyprus. Are you familiar with the country of Cyprus and it's severe monetary crisis from a few years back? It was so bad that the Cyprus government controlled how much money their citizens could take out of their banks. Think about that. That government telling you that you can't access your very own money! How scary is that? Bitcoin offers an alternative. No government (currently) can seize control of your bitcoin, this represents safety and security.
What's the difference between bitcoin and dollars? Bitcoin isn't regulated by any government. It isn't created "out of thin air" as fiat currency is. There will only ever be a finite amount of bitcoin, which controls it's value. Once all bitcoin is mined there will be 21 million in existence, that's it. No one can ever create more (as is done every year with fiat currency). When fiat currency is created by central banks they reduce the value of the existing currency in the market, which results in inflation of prices. This is why the price of every day things (that haven't changed in 50 years, like groceries or utilities) have gone up in price over time. There's a lot of other differences, but it's better for you to read up on them.
Why would I gamble with bitcoin? I don't, but if you want to you can gamble for significantly smaller portions of money (like cents as opposed to dollars or tens of dollars). And you can do it from the convenience of your computer.
Because there are not many reasons why you would want to use Bitcoins instead of Dollars. If you're perfectly happy with using Dollars and don't care about anonymity or centralization, using Bitcoins will only take away from your overall buying/selling/shopping experience because transaction times are rarely as fast as say paying with card or PayPal. There are a few things however that Bitcoin can do better than existing money transfer systems like for example having no country restrictions and no sending/receiving limits. But you can only really appreciate these features if you have a use for them.
Perhaps the most common reason people invest in Bitcoins is because it's more a commodity than a currency which makes it great for speculation. ***