Why can't I just use his unbound ONG - why do I need to buy, to be able to use it

You can imagine that ONG have 3 forms: UNBOUND, CLAIMABLE, NORMAL. You have unbound ONG because you keep ONT in a specific time. Unbound ONG and CLAIMABLE ONG is different from NORMAL ONG.

To use it as normal, first you have to do a transaction to change its status from UNBOUND to CLAIMABLE. Then you have to do another transaction to change CLAIMABLE ONG to NORMAL. Then you can use it with all functions that ONG has.

2 transaction will need 2 transaction fee (ONG)