I just like knowing that I am supporting BB so the developers can keep it going and improving it. It's a small price to pay for the enjoyment I get out of the game. Private leagues are nice because they let me play around with various tactics and line-ups, seeing past pop-ups in training is nice but I don't use the other benefits that much.
But really it is just that I want to keep building my team and playing the game, and by being a supporter I know I am doing my small part to keep things going.