Many Christians do not understand what the Lord wants them to do. If you put the Kingdom of Heaven first, all things will be taken care of for you by the Lord. Distributing tracts allows the Christian, regardless of age or ability, to put the Kingdom first. Essentially, you are placing food on God's table by putting his purposes first. That is a mission we all must pursue.
Tracts are easy to distribute.
There is a serious risk of being turned away by the Lord for not bearing fruit. Salvation is a free gift, yet still, Jesus' warning is a serious issue. Using tracts, you know you are accumulating treasures in Heaven where moth or thief can't destroy. There are other means and methods to accrue treasure by those who have been called to do so. Tracts are only one way and a good way for those who are not experienced to preach.