You may have noticed that every time you send Bitcoin to someone or some address, you’re paying a fee. That fee is the mining fee. 

You pay the mining fee to get your transaction processed. Much like VISA charges to use their network, the Bitcoin network also charges a fee to use its blockchain. 

The difference is that for a VISA credit card transaction, the receiver of the money pays the fee (the store getting your money will pay the fee. They then pass that cost onto you by marking up the goods they sell).

In a Bitcoin transaction, the sender pays the fee instead. This was done by design, since it’s more efficient for the spender to specify how much fee they want to pay. That way the sender (you) can specify a low fee for slower processing or a high fee for faster processing. 

This is due to the fact that miners secure the network. They chose which transactions to process first and which ones to process last or not at all. Obviously the transactions offering the highest fees will be chosen to be processed first and the transactions with the lowest fees will keep getting pushed further down the line until they find a miner willing to accept that low fee. 

This system incentivizes the miners to process transactions (using their computing power and electricity). This also helps senders use the network because, even though you need to pay each time you send money, the cost is often much less than sending money through Western Union or other money transfer services.