Why do I need bitcoin to create Omni transactions?

Omni transactions are regular Bitcoin transactions, which embed data in one way or another. Like any other Bitcoin transaction, Omni transactions also need to pay fees to the miners. Furthermore, some Omni transactions have a very small output to "tag" the recipient of a transaction, which also needs to be accounted for.

The actual cost is based on the transaction fee and the minimum relay transaction fee settings, which can be configured with the txconfirmtarget or paytxfee and minrelaytxfee options.