Answer:
Why is it so easy to weigh fish?
I am needed for most animals and hardcover books
You can see me in water, but I never get wet. What am I?
I am owned by every man though my length differs. Their wives uses me after getting married.
I am a protector. I sit on a bridge. One person can see right through me while others wonder what I hide.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.