People should use bitcoin so that their transactions will be tax free. In some countries, if you earned a big amount of money, the secretary of the tax company will come personally to talk to you and ask you to pay for your 'income' tax. Basically, just use bitcoin so that the government won't take a portion of your money as 'tax'.
That's could being one reason that people use Bitcoin instead of fiat, but don't forget that mostly the bitcoin's community convert its coins into fiat so this they have to pay the tax for this income, and it's not real solution for this issue