How accurate and up to date are the investor's holding data?

We can look up the holdings of various investors on . I am not sure but I think investors are required to disclose their holdings if they own share above certain threshold in any company does that means the data doesn’t shows those holdings which doesn’t cross the threshold? Also why is it required to disclose this information in the first place since it can inflate/deflate the value of shares based on groups blindly following big investors and what is the accuracy of such data?