Probably added them to try to pull in more players without having them to have to pay physical money to access most of the gameplay. And some argue that it was a way to remove RWT. I guess in a sense it sort of did but didn't really impact RWT much.
People who aren't familiar with the RS black market (mostly younger kids) buy bonds for a premium price from Jagex and sell them on the ge. Anyone who is familiar with the RS black market buy the bonds for ridiculously low membership prices ($4 a month, which beats even the legacy membership rates)