Because linear interpolates generally take an alpha value between 0 and 1. Saturate works like a clamp between 0 - 1, so in order to avoid any unwanted negative values coming out of the Lerp, you should clamp the alpha input.
You could always go outside of 0 - 1 but it depends on your use. In most cases, there is a specific purpose to blend between 2 fixed inputs so you clamp the alpha to never get anything outside of the 2 inputs.