最近我正在阅读hadoop源代码，当我阅读OutputFormat类时，checkOutputSpecs方法有这样的文档：
Check for validity of the output-specification for the job.
<p>This is to validate the output specification for the job when it is
a job is submitted. Typically checks that it does not already exist,
throwing an exception when it already exists, so that output is not
overwritten.</p>
我的问题是为什么输出目录不应该存在？
我的猜测是，如果输出目录中有part-****，则可以避免覆盖旧文件。
还有其他原因吗？