Browsing casually their site gives me no clue as to whether you need to give away your copyright when you upload your paper on their site. If I were you I would contact a representative to ask the questions that need to be asked before you can safely give them any of your papers.

Many thanks for your answering.

But why most researchers post working papers on SSRN before submitting them to journals?

what is the benefit of posting papers on SSRN ?

Can I say that those researchers do this to announce that their posted papers on SSRN are conducted by themselves. Therefore, their ideas in posted papers will be protected from being stolen?

Perhaps, yes. Personally I tend not to post manuscripts, but wait until they are published, whence one can post the last internal report version. But I do not even that, most of the time.