Deuteronomy 20:8
And the officers shall speak further unto the people, and they shall say, What man is there that is fearful and fainthearted? let him go and return unto his house, lest his brethren’s heart faint as well as his heart. And the officers shall speak further unto the people, and they shall say, What man is there that is fearful and fainthearted? let him go and return unto his house, lest his brethren’s heart faint as well as his heart.
The officers were required to send home from the army anyone who was fearful and fainthearted. The stated reason was to keep his fear from weakening the courage and hearts of his fellow soldiers, for fear can spread from one person to others and the presence of the fearful would bring only weakness. The order to send home the fearful in Gideon's battle was an application of this law.
The rule showed consideration for family ties. It also recognized the broader principle that one person's thoughts and feelings can influence others and shape their actions. The law meant that those unwilling to face danger and hardship, or distracted by worldly interests with hearts drawn away from God's work, would not strengthen the army and were told to return home so they would not weaken others.
By the same principle, those today who are absorbed in private interests should withdraw from God's work rather than remain to influence others harmfully.