The self-announce procedure relies on the assumption that other pilots are listening. In practice, at small unattended aerodromes, traffic is often sparse. You may be the only aircraft for hours. But you do not know this when you arrive. Another pilot may be in the circuit, unable to see you due to the angle or weather, relying entirely on your radio call to know you are there.
This is why self-announce is a matter of airmanship and safety culture, not regulation. The rule is simple: broadcast your presence. The benefit is enormous: you reduce the risk of collision in an airspace where there is no radar, no ATC, and no official traffic separation.