Is it necessary to use soap before every Wudu and Ghusl, fearing that slight dirt, which becomes apparent after washing the face with water, might act as a barrier preventing water from reaching the skin, even though it is not visible on the face, and the same phenomenon occurs on other parts of the body?

Dirt on the face or body does not prevent water from reaching the skin during ablution (wudu') or ritual bathing (ghusl). It is not obligatory to use soap to remove it; in fact, doing so might be an blameworthy extremism. Sheikh Ibn Uthaymeen stated that washing hands and face with soap during ablution is not legislated; rather, it is a form of going to extremes and being overly meticulous, citing the hadith "Ruined are those who are extreme." However, if there is dirt that cannot be removed except with soap, then there is no harm in using it at that time.