Why exactly do the banks want to go cashless?

As long as there is cash money the banks cannot easily impose negative interests, because then people would simply withdraw their money from the bank accounts. But as soon as there is only virtual money anymore, the banks (and states) have full control over the money: there will be no more way to escape from negative interests.
In addition the state can instruct the banks to freeze all your money immediately - that would strengthen the power of states, governments and banks but harm the freedom of the single individual.
Apart from that it won't be possible any longer to "hide" anything from the banks. You can't buy anything anymore without letting your bank know about it...

Is this to our advantage or to exercise more control?

Some payments may become more comfortable and faster, but in my eyes the disadvantages I named above overbalance that.

Wouldn't it be Great to Use Steem to Buy, Sell And Make Payments?

Crypto currencies like bitcoin and others (and of course I hope steem, too) could play a very important role in case cash money has been eliminated one day. They combine the advantage of being fast and easy to use, but at the same time guarantee the freedom (and independence from the banks) of the individual.
I fear that banks and governments will try to suppress a mass adoption of crypto currencies and possibly will try to forbid it. Nevertheless I hope they will not be able to stop the "trimphal march" of this new kind of "money". :)