Why is this an issue?

StringBuilder is more efficient than string concatenation, especially when the operator is repeated over and over as in loops.

Noncompliant code example

Module Module1
    Sub Main()
        Dim foo = ""
        foo &= "Result: "       ' Compliant - outside of loop

        For i = 1 To 9
            foo &= i            ' Noncompliant
        Next
    End Sub
End Module

Compliant solution

Module Module1
    Sub Main()
        Dim foo = New System.Text.StringBuilder
        foo.Append("Result: ")  ' Compliant

        For i = 1 To 9
            foo.Append(i)       ' Compliant
        Next
    End Sub
End Module