IF, ELSEIF, and When To Use ENDIF

No offense taken. I read about it but there are just contradictions in the ENDIF use. I see rules posted that don't have an ENDIF in a IF THEN statement and they say they work and I also see rules with ELSEIF statements that don't have an ENDIF and they say the rule works as well.
So I agree with the complexity but there is still a question of why do some rules work with it and why do some rules work without it whether it be a IF THEN or IF or ELSEIF?