It separates us from the crowd and identifies our purpose. The signs and banners give us a greater presence than we would have without them.
The signs and banners attract people to us by their content. People actually come to us to inquire and ask questions like...
"Why are you here?"
"What are we doing wrong?"
"What does the sign/banner mean?"
Through these mediums we have more of an opportunity to propagate the gospel message.
Furthermore, these signs/banners continue to promote our message even when we are engaged in one on one conversations. Many of our signs and banners contain Scripture, therefore, God's Word is always being lifted up and exalted.
The message on our signs/banners tend to summarize our message, hence, they often aid us by focusing our preaching. Explaining the signs/banners automatically keeps our attention on men being sinners, lost, undone, and hopeless apart from repentance and faith in the Lord Jesus Christ as their Lord.