As with any sport, hobby or interest, you must feel some accomplishment in order to invest further. Mastery, even in increments, is necessary for continuation.
Similarly, when you put effort into your learning, you desire to see dividends. Reviewing learning with an eye towards mastery develops a cheishek for learning as you become captivated on that sense of accomplishment that comes with each piece you acquire.
Our aim is for baalei batim to further develop their mental taste for learning through that feeling of mastership over the Gemara they learn.