哪位高手能帮小弟我解释一下redim 和 Preserve

Sub Click(Source As Button)
    Dim arr() As String
    Redim arr(1 To 3)
    For i=1 To 3
        arr(i)=i
    Next
    For i=1 To Ubound(arr)
        Msgbox  arr(i)
    Next
    '加上和去掉Preserve 试试就知道了
    Redim Preserve arr(1 To 6) 
    Msgbox Ubound(arr)
    For i=1 To Ubound(arr)
        Msgbox arr(i)
    Next
End Sub

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