Some good reasons why you use bitcoin.
1. It is fast. Transactions are instantaneous and confirmation times are usually done withing 10 minutes.
2. It is cheap to transfer any amount of money anywhere. Fees are insignificant at this point.
3. Your bitcoin are your alone, they can't be blocked by bank, IRS or any institutions.
4. There are no charge backs.
Using bitcoin to transact is much more convenient.
But here's my comment to your post. Fast? Not really. 10 mins confirmation in payment is not that quick. And worse, there are times it goes up to hours. Cheap? Right now transaction fees are simply irrational. They are too significant to just be paid for transaction fees.
The rest I agree. Bitcoin is great but needs development just the same.
Aside from that people choosed to use it because they find it more convenient than using fiat for some expenses or payments, I am not saying this could replace fiat in currency terms but as for what other people say and also in my own experience, even if its not that fast to transact or send money to anyone, you don't have to go to any banks or any markets just to send it, you can freely walk with your own wallet like your a walking bank. Agreed also that because of its transparency there is no other 3rd party user who has access on your account aside from you, so this is also why if keep your credentials in a proper or safe way.