Why am I being asked to add a "transaction fee" when buying eMusic NFTs?

One way to look at Blockchain technology is as a currency of computations. In the analog world, payment is made for physical labor, while in the Blockchain world, payment is made for digital work. For every transaction or interaction with a Smart Contract a small "gas" fee is charged to the user for that computation, and it is paid to the digital miner who mined that block (or made that computation).

Every transaction on the blockchain requires this "gas" transaction fee, regardless of the network that is being used. The amount sent can determine how long it would take for the transaction to be confirmed on the Blockchain network and even if it will be confirmed at all. Note that sending no gas or a very small amount could lead to the transaction not getting confirmed by the blockchain.

Extensive measures are made by the community to ensure that gas prices never get too high, and individual buyers can choose to limit how much gas they are willing to pay.