Why is it growing?
In almost every financial transaction there is an element of trust involved. Before a seller hands over a good or service in exchange for money they need to be trust that the buyer has enough money to pay for the purchase, the same money is not simultaneously being committed for spending elsewhere, and that the payment will be duly made at the agreed time and mode. In the simplest form it could be a small cash transaction in a shop, and this is simple because the cash is visible, and in-person interaction between the two parties allows each one to quickly decide whether they want to trust the other.
However, when payments are to be done in an online mode the whole scenario changes because physical money is no longer visible. This is where a third party intermediary, like a bank or a financial services company comes in. Based on their relationship with each party, the bank acts as the trustee, certifying to the receiver that either the funds have been received, or that the buyer is credit worthy. Either way, the intermediary acts as the trusted broker and is relied upon to complete the transaction. Obviously, this broker has an operational cost that is paid for by one of the transacting parties. This is a significant cost of banking, and these costs are prevalent no matter what is being exchanged, e.g., money, securities, or any other financial instrument.
The draw of blockchain technology is that it offers the security of transacting without needing a third party trustee. With that one characteristic alone it greatly reduces the costs of financial transacting. But apart from this, blockchain technology can be used beyond bitcoin and can be applied for other purposes as well, such as document verification, secure encypted messaging, and many more applications that need authentication of entities.
How is it secure from fraud and misuse?
The nature of public-private key encryption makes it very secure indeed. Fundamentally, a transaction cannot be validated unless hash keys match in sequence along an entire chain. It is this that makes it very difficult to modify key values or duplicate them for fraudulent purposes. In addition, the availability of a public ledger rather than a centralized one also acts as a preventive for attempts to use the same money or document twice.
It is also very difficult to modify existing blockchain records because of the hash chaining. If anyone wanted to tamper with one record they would need to correspondingly modify all records in the chain as well as the records available across the network to maintain integrity, and that would be so difficult to achieve as to be a complete deterrent.
How are miners paid?
Miners contribute servers and processing power for blockchain transactions. If they are a part of a public network they are usually paid in the form of a percentage of the transaction value, at least for bitcoin transactions. For bitcoin transactions, new bitcoin may be generated for them, the quantity calculated in some proportion to the transaction value. The incentive for all miners to receive and store all broadcast validations is that they will not be able to do any further validations unless they are up to date on the previous ones. This also maintains the integrity of the public nature of the blockchain ledger.