传递对象时的byval vs byref

我想检查是否存在具有特定名称的工作表，所以我在下面生成 shtexist 函数。但是，对于 shtexist 中的第二个参数。当我首先通过 byref 时， shtexist(name,thisworkbook) 运行良好，而 shtexist(name,rwb) 没有，它显示 byref 错误。然后我通过它byval，问题解决了。我的问题是为什么在这种情况下 byref/byval 很重要？

Sub update_Click()
Dim updatelist
Dim relname, salname, insname, sname As String
Dim rwb, swb, iwb As Workbook
Dim year, month As Integer
updatelist = ThisWorkbook.Sheets("FrontPage").Range("u2", Range("u2").End(xlDown))
relname = Dir(ThisWorkbook.Path & "/" & "*关系表*.xls?")
Set rwb = Workbooks.Open(ThisWorkbook.Path & "/" & relname)
MsgBox (VarType(ThisWorkbook))
For Each i In updatelist
    sname = CStr(i)
    year = CInt(Left(sname, InStr(sname, ".") - 1))
    month = CInt(Mid(sname, InStr(sname, ".") + 1, 2))
    MsgBox (year & " " & month)
    If shtexist(sname, rwb) Then
        MsgBox ("yes")
    Else
        MsgBox ("no")
    End If
Next

End Sub

Function shtexist(name As String, Optional ByVal wb As Workbook) As Boolean
Dim sht As Worksheet
If wb Is Nothing Then
    Set wb = ThisWorkbook
End If
On Error Resume Next
    Set sht = wb.Sheets(name)
On Error GoTo 0
If sht Is Nothing Then
    shtexist = False
Else
    shtexist = True
End If
End Function