It is very cost effective. In proclamation evangelism it is not always practical to reach every person, so literature fills the gap. People are very curious and want to study and understand new things. Everyone likes free gifts. People will usually line up or even mob you to get the 'free' gift .
Literature is non-threatening.
Some cultures and peoples' personalities do not respond to confrontational evangelism. Some people are shy or scared to talk about their feelings but will listen if God speaks to their heart. The Spirit still supernaturally calls people. Literature evangelism is considered friendly and often leads to discussion on a one on one basis. Using gospel literature means never being sidetracked by rain, snow, electricity.