Prayer is difficult under the best of circumstances. Achieving kavanah (concentration) is a challenge for most of us. Wearing a tallit can help, in much the same way that we dress specially and prepare for other activities.
Rabbi Hayim Halevy Donin offers another interpretation: "How does the mere act of looking upon tzitzit serve to remind one of God's commandments? I suggest that it's like a uniform worn by soldiers in an army. When wearing a uniform, one is especially mindful to whom one owes one's allegiance." (To Pray As a Jew, Harper Collins, 1980)