Why is knowing the percent error is helpful?

Answer 1

it gives an indication of how significant the amount is.
If you're off by 1 out of10 (10%) that's a lot
1 out of 1000 (0.1%) not so much

The absolute error (1) is the same, but the % error matters more.

The best first place to try on general questions like this is google.
You will find many examples, illustrations, discussions, and videos.

Answer 2

Oooo thank you oobleck!!!