Sildenafil and tadalafil work well for many men, but not for everyone. Some do not respond, some cannot tolerate the side effects, and some find the ongoing cost hard to sustain. Compounding lets your prescriber order a different strength, a combination of active ingredients, or a different delivery form such as a troche that dissolves under the tongue.
Any treatment starts with your health history and a look at everything else you take, because ED medications interact seriously with nitrates and with some blood pressure drugs. It is also worth knowing that ED is frequently an early sign of cardiovascular disease or diabetes, so it is a symptom worth investigating rather than simply working around.