TTC and charting question/multiple preg test?

I am just curious, I have been looking at sample charts online and I often notice that once a person gets a BFP, they continue taking tests for days after. I can see taking one more just to be sure but why take 5-6 tests? Is there a reason? I was thinking once I get a BFP, I would just call my dr. but should I take multiple tests first? Thanks!