Why is this an issue?

If a local variable is declared but not used, it is dead code and should be removed. Doing so will improve maintainability because developers will not wonder what the variable is used for.

Noncompliant code example

Public Function NumberOfMinutes(ByVal hours As Integer) As Integer
    Dim seconds As Integer = 0 ' Seconds never used
    Return hours * 60
End Function

Compliant solution

Public Function NumberOfMinutes(ByVal hours As Integer) As Integer
    Return hours * 60
End Function

Exceptions

Unused locally created resources in a Using statement are not reported.

Using t = New TestTimer()
End Using