Why is it important to have at least two benchamrks(BM) on constructions sites

I am currently taking a surveying class in college and need help with this question. Why were you advised to have at least two benchmarks(BM) on construction sites?

The first two answers may be perfectly correct, but my first thought was redundancy - if one of the bench marks is destroyed by accident, it can be re-established from the other one.

Sometimes there are features that block the first benchmark so if you have a second, its backup. For example, a building is in your way or the peak of a hill.

you also need two benchmarks to give you a base line.