Why a person who has to take a HIV test after 6 weeks to 3 months is told not to have within these days...?

within these days means within 6 weeks to 3 months after the unprotected . Is it because that person may infect his/her partner or is it because the result of the HIV test might come wrong if the person have before he/she is ready for the test? (The HIV antibody test looks for antibodies to the virus in a person's blood. For most people these antibodies take 6 weeks to 3 months to develop.)