1. They offer ease of shopping for your customer because they accept credit cards. Huge amounts of transactions occur online, and you need to be able to take advantage of that. 2. They all have tutorials and instructions on how to set them up, and you don't have to use just one, but can have a number of them. 3. If you have any problems with fraud etc. they will handle the situations for you. 4. You can use your own bank account for receiving payment.
Setting up a payment processor:
Visit the home page of any of these payment processors and you can see their tutorials and instructions on set up There are many different tools offered.