People should use bitcoin for many reason. From the perspective of users it can be various things can be said which might be harmfull for mankind so should word doesnt set that good to answer the question. Perhaps i will say why people use bitcoin?
Investor: investorbased people sees btc as a form of opportunities to earn some profit by trading or exchanging in the exchange house. They consider btc as an form of assets which have the highest attention of people this days. And the frowth rate is so exception for any asset which makes people to feel interest about it.
Criminal or corruption: for drug dealing or any kind of bad worn btc is a blessing to them so they can be appear as hollow man in a sense of money trafficking. Btc help bad money to look good. Anyonr can turn his or her black money into white money.
Easy drug dealing arms dealing can be done. So why not to use it.
In both case for an investor or criminal btc is a good platform for them to get their job done.