TThreadList问题

TThreadList的用法中：
The following example shows how to use the LockList and UnlockList methods to access a thread-safe version  of a TList object:with MyThreadList.LockList do  try
    for X := 0 to Count-1 do
      Something(Items[X]);
  finally
    MyThreadList.UnlockList;
  end;请问为什么要MyThreadList.UnlockList，我写一个程序少了这句就出现问题了，我是菜鸟，有点不好意思问这种问题，呵呵～