Question

why does your backstop need to be safe and solid when shooting?
to have protection from the sun
to capture bullets and prevent ricochets
to have a spot to set up camp
to keep you hidden from game

Explanation:

A backstop in shooting (like in firearm use) is designed to stop bullets and stop them from ricocheting (bouncing off and potentially causing harm). Protection from the sun, setting up camp, or hiding from game are not related to a backstop's purpose. Capturing bullets and preventing ricochets matches the safety role of a backstop.

Answer:

To capture bullets and prevent ricochets