Is the plastic shield on my 2007 prius under the front bumper necessary?

If u dont find a way to keep it up dirt and all kinds of things can get up there best to fix it zipties or something like that

Is it a shield, or is it a thin strip air dam? The shields under the engine prevent water a debris from fouling the belts and the engine. The thin black strip directly under the bumper is for aerodynamics at upper speeds, and also can help direct airflow into the radiator. In either case, I would try to re-attach it. Then,.... be careful when pulling up to parking blocks in parking lots. It probably got damaged when it scraped one there. You pull in, hear the crunch, and then back away. By this time the damage is done. These old blocks were designed ages ago for high riding older vehicles. It's a shame that nobody has updated these things to the lower clearances of newer vehicles.