Hi Hosein,
Thanks for your report!
Without knowing what you have tried where it is difficult to say what you want to report as a bug 
However, I think you have a different idea of what it does. Unfortunately, we haven’t documented this anywhere yet.
We use the wc_customer_bought_product core function, which does the following:
“Checks if a customer has bought an item. The check can be done by email or user ID or both.”
Useful core functions - Woo Developer Docs
In other words, it checks whether you have purchased a product as a logged-in user to show you other relevant products or simply a note that you have already purchased it.
Best regards,
timmse