The correct answer to this open question is the following.
If legislators weren't afforded immunities, how would the democratic process suffer?
US democracy would suffer in that American legislators could not speak their truth in order to defend the interests of the citizens. When doing their jobs, US Congressmen have to make tough decisions, many times affecting private agendas or economic interests of people or corporations. Without immunity, legislators cold get a lawsuit. Congressmen are the representatives of the people and have to fight to solve people's problems. And that sometimes means taking serious risks against powerful millionaires, corporations, and political groups.