They also want to mitigate the risk that the team will fall short of expectations, leading to a decline in single ticket/walk-up sales. You want to sell future tickets when expectations for the team are sky-high.
I don't do ticket plans, because I like being able to make spur of the moment decisions about which games I'll attend or not attend. If they sold something like a pre-paid card with 10 tickets on them, that I can use like a SmartTrip card to enter games that aren't sold out, I would totally do that.