That just leaves the code in a state where it's not clear what's wrong with it, because it works most of the time. There are just two types of code: obviously correct, or not obviously wrong.

If you write some code and you don't know how it works, it's most likely not correct in some edge cases.

Code that is written to be so simple it's obviously doing the right thing is better than battle tested mess of if statements.

I've given interviews of fizzbuzz and even though developers usually recognize they need a separate case for 15, they don't always do the else ifs right (maybe test for the 15 case last when you've already hit the divisible by 3 case first, or similar silly mistakes). In that case, doing the string accumulation refactor makes it clear that the code has no mistakes because it for sure adds a Fizz first and a Buzz second in the 15 case. It's also a good refactor for extending to Bazz (divisible by 7) as well, so it's something you will want to do when you want to add that feature